Organization and Description of Business	12 Months Ended
Organization and Description of Business	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Description of Business	1. 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 Milpitas, California. The Company is a clinical-stage biopharmaceutical company with a proprietary peptide technology platform focused on discovering and developing new chemical entities to address significant unmet medical needs. Protagonist Pty Ltd is a wholly-owned subsidiary located in Brisbane, Australia. The Company manages its operations as a single operating segment. *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 split. All issued and outstanding common stock, options to purchase common stock and per share amounts contained in the consolidated financial statements have been retroactively adjusted to reflect the reverse stock split for all periods presented. The consolidated financial statements have also been retroactively adjusted to reflect a proportional adjustment to the conversion ratio for each series of preferred stock in connection with the reverse stock split. *Initial Public Offering* On August 10, 2016, the Company’s registration statement on Form S-1 (File Nos. 333-212476 and 333-213071) relating 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 and there are no shares of redeemable convertible preferred stock outstanding. In September 2016, the Company issued an additional 252,972 shares of 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, and after deducting offering costs paid by the Company. *Liquidity* The Company has incurred net losses from operations since inception and has an accumulated deficit of $64.6 million as of December 31, 2016. The Company’s ultimate success depends on the outcome of its research and development activities. The Company expects to incur additional losses and negative cash flows for the foreseeable future and it anticipates the need to raise additional capital to fully implement its business plan. The Company intends to raise such capital through the issuance of additional equity and/or strategic alliances with partner companies. As of December 31, 2016, the Company had $87.7 million of cash, cash equivalents and available-for-sale securities and management believes the existing cash, cash equivalents and available-for-sale securities will be sufficient to meet the Company’s anticipated operating and capital expenditure requirements.