Thomson arrived in Llanelli from Super Rugby side Hurricanes in 2018 and the strength of his performances in west Wales have earned him 10 Scotland caps.
In total Thomson has made 31 appearances for Scarlets while he also played at the 2019 Rugby World Cup.
Thomson, 30, said: “I am delighted to have signed a new contract with the Scarlets.
“Ever since myself and my family have arrived here we have been made to feel welcome by everyone at the Scarlets, team-mates, staff and supporters.
“It has been a tough season for a number of reasons, but there is a huge amount of quality in the squad – experienced internationals and talented young players who have stepped up and made their mark.
“I am excited by what this group can produce in the coming years.”
Thomson is the latest international to have re-signed for the Scarlets, following in the footsteps of Wyn Jones, Aaron Shingler and Ryan Elias.
Highly-rated youngsters Carwyn Tuipulotu, Morgan Jones and prop Javan Sebastian are also committed to the Llanelli cause.
Scarlets head coach Glenn Delaney said: “It is fantastic Blade has signed a new deal with the Scarlets.
“Blade is a real athlete, a player who can produce game-changing moments as he showed in our derby win over the Ospreys over Christmas.
“He has proven himself at international level, is a player who has a lot of experience in Super Rugby and has grown into an important and influential member of our squad”