UFC officials broke the news on Monday. The championship bout will take place inside Sleep Train Arena in Sacramento, Calif.
Pettis (17-2) claimed the title last month at UFC 164, defeating Ben Henderson via first-round submission. Original plans called for his first defense to come against T.J. Grant, who he replaced at UFC 164 when Grant withdrew due to injury.
It appears, however, that Grant (21-5) is still feeling the effects of the concussion that forced him from the title opportunity last month. The 29-year-old contender revealed on Twitter he failed to receive medical clearance for the December bout.
"Unfortunately, I won't be fighting Pettis on Dec. 14," Grant posted. "I am still not yet 100 percent and I can't commit to fight. UFC is going with someone else instead."
That someone will be Thomson (20-5), who recorded a blowout TKO victory over Nate Diaz at a UFC on Fox event in April. It was Thomson's first bout inside the UFC since August 2004. He fought for several promotions during that absence, including Strikeforce, where he won the lightweight belt in 2008.
Thomson is currently ranked the No. 7 lightweight in the world, according to ESPN.com.
It's unknown how long Grant will be sidelined or whether he'll remain the No. 1 contender at 155 pounds when he returns. In another Twitter post, Grant stated if he had to accept another non-title fight when healthy, he would.