They will have to clearly state if they have been paid or received any gifts or loans of products they endorse.
It follows warnings from the Competition and Markets Authority that their posts could break consumer law.
Online endorsements can boost brands but can also mislead, said the CMA.
The CMA has not made a finding on whether the influencers named breached consumer law, but said all of them volunteered to change their practices following an investigation.
However, if they fail to comply with the agreement reached with the CMA, they could be taken to court and face heavy fines or prison sentences of up to two years.
The celebrities named by the CMA are:
- Pop star Rita Ora
- Actress Michelle Keegan
- Singer Ellie Goulding
- Writer and model Alexa Chung
- Designer and model Rosie Huntington-Whiteley
- Video blogger Zoe Sugg (who vlogs under the name Zoella)
- Mario Falcone from The Only Way Is Essex
- Alexandra 'Binky' Felstead from Made In Chelsea
- Holly Hagan from Geordie Shore
- Model Iskra Lawrence
- Camilla 'Milly' Macintosh from Made In Chelsea
- Reality TV personality Megan McKenna
- Chloe Sims from The Only Way Is Essex
- Louise Thompson from Made In Chelsea
- Fashion video blogger James Chapman (who vlogs as Jim Chapman)
- Fashion blogger Dina Torkia
Warning letters have been sent to a number of other unidentified celebrities, urging them to review their social media posts.