Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2020	Nov. 07, 2020
Cover [Abstract]
Amendment Flag	false
Document Type	10-Q
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Entity Registrant Name	PLIANT THERAPEUTICS, INC.
Entity Central Index Key	0001746473
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		35,479,853
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Trading Symbol	PLRX
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Security Exchange Name	NASDAQ
Entity File Number	001-39303
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
Document Quarterly Report	true
Document Transition Report	false