"Twitter has been an amazing ride, but as I mentioned internally a few weeks back, my time is coming to an end. I’m confident to leave the program with an amazing security team," Coates said on Twitter on Wednesday.
Coates' departure comes amid a growing scandal involving fellow social network Facebook and the exploitation of users' data by third parties.
The Twitter information security chief, who has been with the company for three years, is planning to launch a security start-up, the details of which he promised to reveal soon.
On Monday, the director of the Information Security Engineering at Google, Michal Zalewski, said he would be leaving the company after spending 11 years there.
All three digital giants have been facing increasing pressure over their content policy, including the spread of fake news, especially during the election.