It had been suggested England might move on from 35-year-old wicketkeeper Buttler in the shortest format but he will retain the gloves for the series against the world champions next month.
The 22-year-old Coles has been selected after the spin-allrounder put in some impressive performances in franchise cricket around the world and for England Lions in recent seasons.
"We have selected a larger squad to accommodate for the fact that this series will begin shortly after the Test series against New Zealand is due to conclude which allows us to be flexible," Marcus North, England's new national selector, said in a statement.
"James Coles is an exciting addition and has earned his place following his performances with the England Lions and in T20 competitions here and abroad during the past 12 months."
The series against India marks the start of a new T20I cycle for England, leading into the Olympics (where England players are likely to dominate the Team GB squad) and the T20 World Cup in 2028.
England T20I squad: Harry Brook (captain), Rehan Ahmed, Jofra Archer, Sonny Baker, Tom Banton, Jacob Bethell, Jos Buttler (wk), James Coles, Jordan Cox, Sam Curran, Liam Dawson, Will Jacks, Saqib Mahmood, Adil Rashid, Phil Salt, Josh Tongue, Luke Wood
Ben Stokes and Gus Atkinson have been cleared to return to England's squad for the third and deciding Test against New Zealand, after both players received written warnings following an investigation into a breach of team protocol.
England's hopes of pulling off the highest successful run chase in Test history were hanging by a thread at The Oval on Saturday, with Joe Root once again carrying the fight against New Zealand.
Henry Nicholls struck an unbeaten century as New Zealand tightened their grip on the second Test against England at The Oval, ending day three with a commanding 352-run lead and seven wickets still in hand.
England face an uphill battle in the second Test against New Zealand at The Oval after a difficult second day left them well behind in the contest despite a promising start to the match.
