Jack Walker and Max Wright joined the 23-year-old flanker in finalising extensions ahead of Saturday's (AEDT) European Champions Cup clash with Harlequins.
Underhill has made 15 appearances for England and started the 2019 Rugby World Cup final loss to South Africa in November.
"I am delighted to have re-signed with the club. I am excited for my future with Bath Rugby. It's a club that has a lot of potential. It has been a new set-up this year, and I have been impressed by how everything is gelling together," Underhill said.
"We have a very young squad with huge potential, and we have a long way to go but that is something to look forward to and I very much want to be a part of it.
"We have a young pack and some really promising youngsters coming through the pathway and added to that a bit of experience and I think it's an exciting blend in the squad."
Bath is sixth in the Premiership with four wins from eight games, while it has been eliminated from the European Champions Cup.