"It was obviously a really tough decision" Venus said.
"Ryan is top 50 in singles and that's his priority and he wants to continue to develop that so it makes it tough to build both."
During the season Harrison and Venus seldom practised together at tournaments, unless the American lost early in singles. Most of the leading doubles teams are specialists.
Venus says Harrison has been extremely supportive and the duo remain best friends off the court.
"I had been trying to figure out what's the best thing to do and Ryan advised me to test the waters and see who's out there and what possibilities I had. He said if I didn't have anything we could keep playing until something worked out." Venus said.
"I had heard that Raven and his partner [Rajeev Ram] were parting ways so I flicked him a message and it all fell into place from there."
Klaasen, 35, has won 13 ATP doubles titles, including in Auckland in 2015, and the prestigious Indian Wells Masters this year with Ram.
Venus will leave for Brisbane after Christmas for the season-opening Brisbane International before returning home for the ASB Classic ahead of the Australian Open.