The winger was top scorer on the British and Irish Lions tour to New Zealand in 2017 and has also scored 19 times for Scotland including a hat-trick against Fiji in this year's Autumn Test window.
The news means that the 30-year-old will extend his stay at Scotstoun to nine seasons, having joined from Ulster in the summer of 2011.
He has since gone on to play 122 times for the club and is the second highest try scorer in Glasgow's history. His 39 scores bettered only by DTH van der Merwe.
Seymour said: "It's a decision I'm very happy with. My family is very happy in Glasgow and it's a club I feel a massive part of. Among the coaching staff and players I've made some great friends here and I can't really imagine being anywhere else.
"There are so many things Glasgow as a club has enabled me to do in my career and I can't thank them enough. I'm really proud and happy to be able to say I've been a part of this club for as long as I have."
Glasgow Warriors Head Coach Dave Rennie said: "We're absolutely rapt that Tommy has re-committed. He's very highly regarded and has been an important part of the fabric here for a number of years.
"His skillset makes him a crucial member of our back three. He is world class under the high ball and has electric pace. He's a good man and we're pleased that he sees his future in Glasgow."