Negredo will receive an annual salary of 4.5 million euros ($5.3 million), according to sports daily Fanatik.

The daily said Valencia, who had lent the player to Middlesbrough, would receive 2.5 million euros ($2.9 million).

The 31-year-old striker is expected to arrive in Istanbul later on Thursday before a medical check while the transfer will be made official on Friday, Fanatik said.

Former Manchester City player Negredo has 21 caps with the Spanish national team.

He had a mixed season at Middlesbrough, scoring nine goals in the Premier League, but was not able to avoid the north-eastern England team's relegation to the Championship.

The transfer comes a month after the Black Eagles signed Portuguese international defender Pepe on a two-year deal.

For years in the shadow of rival Istanbul teams Fenerbahce and Galatasaray, Besiktas won the Super Lig title for the last two seasons, claiming Turkey's sole automatic Champions League spot.