Cover - shares	3 Months Ended
	Mar. 31, 2021	May 06, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
Document Transition Report	false
Entity File Number	001-39303
Entity Registrant Name	PLIANT THERAPEUTICS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-4272481
Entity Address, Address Line One	260 Littlefield Avenue
Entity Address, City or Town	South San Francisco,
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	481-6770
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Trading Symbol	PLRX
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding (in shares)		35,894,459
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Central Index Key	0001746473
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1