The Rockets acquired the 2017 NBA MVP from the Oklahoma City Thunder on Thursday in exchange for Chris Paul, protected first-round draft picks in 2024 and 2026, and the rights to swap picks in two other drafts.
Harden, who played his first three NBA seasons in Oklahoma City with Westbrook, is pleased to be reunited with his old team-mate.
"When I left OKC I was sad," Harden said on Friday, via The Athletic.
"Then Houston made a home for me. Rockets is my home. And now an opportunity came available where Russ wasn't happy. Now we're back. It's going to be a lot of fun this year, I guarantee you that."
According to the Houston Chronicle, Harden and Westbrook "strongly favoured the move and a chance to play in a backcourt together".
Now, it is up to head coach Mike D'Antoni to build the offense around the two NBA MVPs, an award Harden won in 2018.