Ellis will be guaranteed at least $6 million, according to the reports.
The NFL Network first reported the agreement and its terms.
Ellis was slated to become an unrestricted free agent next week before the Raiders decided to keep the stout run-stopping defensive tackle.
The 6-foot-2, 335-pound Ellis is more gap-filling run-stopper than interior pass-rushing menace, as his career-high 48 tackles attest, though he did also get his first half-sack, when he teamed with Mario Edwards Jr. to take down Titans quarterback Marcus Mariota in the season opener.
Ellis, a fourth-round pick of the Raiders out of Louisiana Tech who does not turn 28 until Dec. 27, is known as "Jelly" and can also play nose tackle in a 3-4 scheme. He played all 16 games for the third time in his four-year career and started a career best-tying 14 times.
He had a season-high six tackles at Washington in Week 3.
ESPN's Paul Gutierrez and The Associated Press contributed to this report.