A person familiar with the deal says it's a five-year contract worth $21.1 million, including $12.32 million guaranteed. The person spoke to The Associated Press on condition of anonymity because the deal had not yet been signed.
A Seahawks spokesman confirmed the agreement on Saturday night but wouldn't reveal the terms.
Seattle selected the 21-year-old Thomas out of Texas with the No. 14 pick in the April draft. He missed two practices on the first day of training camp.
Left tackle Russell Okung, drafted by the Seahawks with the No. 6 selection, remains unsigned.