Regulatory Matters: Schedule of Compliance with Regulatory Capital Requirements for Mortgage Companies (Details) $ in Thousands	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)
Timberland Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Leverage Capital, Actual Amount	$ 247,162	$ 229,206
Tier 1 Leverage Capital, Actual Ratio	0.125	0.120
Tier 1 Leverage Capital, Regulatory Minimum To Be Adequately Capitalized, Amount	$ 79,319	$ 76,319
Tier 1 Leverage Capital, Regulatory Minimum To Be Adequately Capitalized, Ratio	0.040	0.040
Tier 1 Leverage Capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 99,148	$ 95,399
Tier 1 Leverage Capital, To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	0.050	0.050
Tier 1 Risk-based Capital Ratio, Common Equity, Actual Amount	$ 247,162	$ 229,206
Tier 1 Risk-based Capital Ratio, Common Equity, Actual Ratio	19.30%	18.00%
Tier 1 Risk-based Capital Ratio, Regulatory Minimum To Be Adequately Capitalized, Amount	$ 57,623	$ 57,318
Tier 1 Risk-based Capital Ratio, Regulatory Minimum To Be Adequately Capitalized, Ratio	4.50%	4.50%
Tier 1 Risk-based Capital Ratio, Regulatory Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 83,234	$ 82,793
Tier 1 Risk-based Capital Ratio, Regulatory Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%	6.50%
Tier 1 Risk-based Capital, Actual Amount	$ 247,162	$ 229,206
Tier 1 Risk-based Capital, Actual Ratio	0.193	0.180
Tier 1 Risk-based Capital, Regulatory Minimum To Be Adequately Capitalized, Amount	$ 76,831	$ 76,424
Tier 1 Risk-based Capital, Regulatory Minimum To Be Adequately Capitalized, Ratio	0.060	0.060
Tier 1 Risk-based Capital, Regulatory Minimum To Be Well Capitalized, Amount	$ 102,442	$ 101,899
Tier 1 Risk-based Capital, Regulatory Minimum To Be Well Capitalized, Ratio	0.080	0.080
Total Capital, Actual Amount	$ 263,200	$ 245,152
Total Capital, Actual Ratio	0.206	0.193
Total Capital, Regulatory Minimum To Be Adequately Capitalized, Amount	$ 102,442	$ 101,899
Total Capital, Regulatory Minimum To Be Adequately Capitalized, Ratio	0.080	0.080
Total Capital, Regulatory Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 128,052	$ 127,374
Total Capital, Regulatory Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	0.100	0.100
Timberland Bancorp
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Leverage Capital, Actual Amount	$ 248,755	$ 231,092
Tier 1 Leverage Capital, Actual Ratio	0.126	0.121
Tier 1 Risk-based Capital Ratio, Common Equity, Actual Amount	$ 248,755	$ 231,092
Tier 1 Risk-based Capital Ratio, Common Equity, Actual Ratio	19.40%	18.10%
Tier 1 Risk-based Capital, Actual Amount	$ 248,755	$ 231,092
Tier 1 Risk-based Capital, Actual Ratio	0.194	0.181
Total Capital, Actual Amount	$ 264,800	$ 247,044
Total Capital, Actual Ratio	0.207	0.194

Tier One Risk Adjusted Capital

No definition available.

Name:	tsbk_TierOneRiskAdjustedCapital
Namespace Prefix:	tsbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tier One Risk Adjusted Capital Required For Capital Adequacy

+ References

No definition available.

+ Details

Name:	tsbk_TierOneRiskAdjustedCapitalRequiredForCapitalAdequacy
Namespace Prefix:	tsbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tier One Risk Adjusted Capital Required For Capital Adequacy To Risk Weighted Assets

+ References

No definition available.

+ Details

Name:	tsbk_TierOneRiskAdjustedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	tsbk_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier One Risk Adjusted Capital Required To Be Well Capitalized

+ References

No definition available.

+ Details

Name:	tsbk_TierOneRiskAdjustedCapitalRequiredToBeWellCapitalized
Namespace Prefix:	tsbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tier One Risk Adjusted Capital Required To Be Well Capitalized To Risk Weighted Assets

+ References

No definition available.

+ Details

Name:	tsbk_TierOneRiskAdjustedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	tsbk_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier One Risk Adjusted Capital To Risk Weighted Assets

+ References

No definition available.

+ Details

Name:	tsbk_TierOneRiskAdjustedCapitalToRiskWeightedAssets
Namespace Prefix:	tsbk_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of total capital as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_Capital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of minimum total risk-based capital required for capital adequacy as defined by regulatory framework.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1B

+ Details

Name:	us-gaap_CapitalRequiredForCapitalAdequacy
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Ratio of minimum total risk-based capital to risk-weighted assets required for capital adequacy as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of minimum total risk-based capital categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_CapitalRequiredToBeWellCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Ratio of minimum total risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of total risk-based capital to risk-weighted assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_CapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1B

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1

+ Details

Name:	us-gaap_ComplianceWithRegulatoryCapitalRequirementsUnderBankingRegulationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of Tier 1 leverage capital as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of minimum Tier 1 leverage capital required for capital adequacy as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Ratio of minimum Tier 1 leverage capital to average assets required for capital adequacy as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of minimum Tier 1 leverage capital categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Ratio of minimum Tier 1 leverage capital to average assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of Tier 1 capital to average assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of Tier 1 risk-based capital as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of minimum Tier 1 risk-based capital required for capital adequacy as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Ratio of minimum Tier 1 risk-based capital to risk-weighted assets required for capital adequacy as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of minimum Tier 1 risk-based capital categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Ratio of minimum Tier 1 risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of Tier 1 risk-based capital to risk-weighted assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	dei_LegalEntityAxis=tsbk_TimberlandBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=tsbk_TimberlandBancorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: