Colorado will pay a fee of $10.7 million (£7.9m), sources add. Whittaker is flying to the United States on Wednesday after completing a medical in London on Tuesday.
The Rapids previous club-record is $7 million (£5.2 million) to sign American attacker Paxten Aaronson.
Whittaker joined Boro for a fee of around $9.5 million (£7 million) in January 2025 from Plymouth Argyle. He excelled last season with 14 goals and seven assists in 40 appearances.
Whittaker will join a Rapids team led by Matt Wells. The English coach’s team is undergoing an attacking overhaul this summer with an emphasis on wide play, with Whittaker viewed as the main man when he arrives and debuts.
The Rapids evaluated at a number of top targets in Europe to sign a DP winger, sources say. The club will have two DPs when Whittaker arrives, alongside Aaronson
The team transferred center back Lucas Herrington to Hull City this summer for an initial fee of $17 million, one of the largest in MLS history.
Colorado has been solid under Wells in his first year, in seventh place in the Western Conference with 31 points after 22 matches. The team has been strong defensively, with five consecutive clean sheets, but has lagged a bit in attack.