Next year’s would-be biggest free agent is getting signed before Bryce Harper.

Nolan Arenado and the Rockies are finalizing an eight-year contract extension worth $260 million, The Post confirmed, which would make the outstanding third baseman the owner of the highest average salary of any position player in baseball at $32.5 million. The deal includes an opt-out after three seasons and a no-trade clause, ESPN reported, and replaces his final, record-setting payday for an arbitration-eligible player of $26 million this season.

Detroit’s Miguel Cabrera previously was the best-paid-by-year position player at $31 million, but Arenado will not be challenging Giancarlo Stanton’s $325 million for highest total or Zack Greinke’s $34.42 million for highest average for any player. It also will set a new bar for Harper to beat if he wants to take away the title for highest position-player average, as he flirts with the Phillies, Giants and Dodgers, all offering various structures for deals, some guaranteeing the length he wants, some guaranteeing the average he wants.

The Yankees were believed to be strong suitors for Arenado if the 27-year-old hit the market next season. The four-time All-Star and six-time Gold Glover is the type of two-way, non-controversial superstar that Harper and Manny Machado aren’t, and the Yankees are fielding a still-growing and still-learning Miguel Andujar at third.