The allrounder only featured in one-day cricket for the Bears in his four-year stint at Edgbaston
Thomason only featured in one-day matches for the Bears during his four-year spell at Edgbaston and has signed a contract at Hove for an undisclosed period of time.
The 21-year-old made 17 appearances in List A cricket for Warwickshire and also featured 28 times for Birmingham Bears in the T20 Blast.
Birmingham-born Thomason said: "This is a lovely part of the country to live in. I'm looking forward to getting stuck in.
Thomason has ended his four-year playing spell at Edgbaston
"This is a great opportunity for me. The club has some great players who I am sure I will learn lots from."
Sussex head coach Jason Gillespie added: "We identified that we want to strengthen our seam-bowling depth and, as a talented fast-medium bowling all-rounder, Aaron fits the bill perfectly.
"He's come through the youth system at Warwickshire and from what we've seen we think we can help him continue his development as a cricketer."
Thomason made 17 appearances in List A cricket during his time with Warwickshire, with his debut coming against Middlesex at Lord's aged just 17 in 2014.
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