Organization and Description of Business	9 Months Ended
Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Description of Business	Organization and Description of Business Protagonist Therapeutics, Inc. (the “Company”) was incorporated in the state of Delaware on August 22, 2006 and is headquartered in Newark, California. The Company is a clinical-stage biopharmaceutical company with a proprietary technology platform which is utilized to discover and develop novel peptide-based drugs to address significant unmet medical needs. Protagonist Pty Limited (“Protagonist Australia”) is a wholly-owned subsidiary of the Company and is located in Brisbane, Australia. Protagonist Australia was incorporated in Australia in September 2001. The Company became the parent of Protagonist Australia pursuant to a transaction in which all of the issued and outstanding capital stock of Protagonist Australia was exchanged for shares of the Company’s common stock and Series A preferred stock. The Company manages its operations as a single operating segment. Liquidity The Company has incurred net losses from operations since inception and has an accumulated deficit of $98.5 million as of September 30, 2017. The Company’s ultimate success depends on the outcome of its research and development activities. The Company expects to incur additional losses in the future and it anticipates the need to raise additional capital to fully implement its business plan. In September 2017, the Company filed a registration statement on Form S-3 with the Securities and Exchange Commission (“SEC”) which permits the offering, issuance, and sale by the Company of up to a maximum aggregate offering price of $200.0 million of its common stock. Up to a maximum of $50.0 million of the maximum aggregate offering price of $200.0 million may be issued and sold pursuant to an At-The-Market financing facility under a sales agreement with Cantor Fitzgerald & Co. As of September 30, 2017, the Company had not sold any securities pursuant to the shelf registration statement. The Company will also continue to seek funds through equity or debt financings, collaborative or other arrangements with corporate sources, or through other sources of financing, but there is no assurance that such financing will be available at terms acceptable to the Company, if at all. Initial Public Offering On August 10, 2016, the Company’s registration statement on Form S-1 (File Nos. 333-212476 and 333-213071) related to its initial public offering (“IPO”) of common stock became effective. The IPO closed on August 16, 2016, at which time the Company issued 7,500,000 shares of its common stock at a price of $12.00 per share. In addition, upon closing the IPO, all outstanding shares of the redeemable convertible preferred stock converted into 8,577,571 shares of common stock. There were no shares of redeemable convertible preferred stock outstanding at September 30, 2017 or December 31, 2016. In September 2016, the Company issued an additional 252,972 shares of its common stock at a price of $12.00 per share following the underwriters’ exercise of their option to purchase additional shares. The Company received an aggregate of $83.6 million in cash, net of underwriting discounts and commissions, after deducting offering costs paid by the Company. Reverse Stock Split In July 2016, the Company’s board of directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse split of the Company’s issued and outstanding common stock at a 1-for-14.5 ratio, which was effected on August 1, 2016. The par value and authorized shares of common stock and convertible preferred stock were not adjusted as a result of the reverse stock split. All issued and outstanding common stock, options to purchase common stock and per share amounts contained in the condensed consolidated financial statements have been retroactively adjusted to reflect the reverse stock split for all periods presented.

Organization and Description of Business

Protagonist Therapeutics, Inc. (the “Company”) was incorporated in the state of Delaware on August 22, 2006 and is headquartered in Newark, California. The Company is a clinical-stage biopharmaceutical company with a proprietary technology platform which is utilized to discover and develop novel peptide-based drugs to address significant unmet medical needs. Protagonist Pty Limited (“Protagonist Australia”) is a wholly-owned subsidiary of the Company and is located in Brisbane, Australia. Protagonist Australia was incorporated in Australia in September 2001. The Company became the parent of Protagonist Australia pursuant to a transaction in which all of the issued and outstanding capital stock of Protagonist Australia was exchanged for shares of the Company’s common stock and Series A preferred stock. The Company manages its operations as a single operating segment.

Liquidity

The Company has incurred net losses from operations since inception and has an accumulated deficit of $98.5 million as of September 30, 2017. The Company’s ultimate success depends on the outcome of its research and development activities. The Company expects to incur additional losses in the future and it anticipates the need to raise additional capital to fully implement its business plan.

In September 2017, the Company filed a registration statement on Form S-3 with the Securities and Exchange Commission (“SEC”) which permits the offering, issuance, and sale by the Company of up to a maximum aggregate offering price of $200.0 million of its common stock. Up to a maximum of $50.0 million of the maximum aggregate offering price of $200.0 million may be issued and sold pursuant to an At-The-Market financing facility under a sales agreement with Cantor Fitzgerald & Co. As of September 30, 2017, the Company had not sold any securities pursuant to the shelf registration statement. The Company will also continue to seek funds through equity or debt financings, collaborative or other arrangements with corporate sources, or through other sources of financing, but there is no assurance that such financing will be available at terms acceptable to the Company, if at all.

Initial Public Offering

On August 10, 2016, the Company’s registration statement on Form S-1 (File Nos. 333-212476 and 333-213071) related to its initial public offering (“IPO”) of common stock became effective. The IPO closed on August 16, 2016, at which time the Company issued 7,500,000 shares of its common stock at a price of $12.00 per share. In addition, upon closing the IPO, all outstanding shares of the redeemable convertible preferred stock converted into 8,577,571 shares of common stock. There were no shares of redeemable convertible preferred stock outstanding at September 30, 2017 or December 31, 2016. In September 2016, the Company issued an additional 252,972 shares of its common stock at a price of $12.00 per share following the underwriters’ exercise of their option to purchase additional shares. The Company received an aggregate of $83.6 million in cash, net of underwriting discounts and commissions, after deducting offering costs paid by the Company.

Reverse Stock Split

In July 2016, the Company’s board of directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse split of the Company’s issued and outstanding common stock at a 1-for-14.5 ratio, which was effected on August 1, 2016. The par value and authorized shares of common stock and convertible preferred stock were not adjusted as a result of the reverse stock split. All issued and outstanding common stock, options to purchase common stock and per share amounts contained in the condensed consolidated financial statements have been retroactively adjusted to reflect the reverse stock split for all periods presented.