Bruce Jenner is dating one of Kris Jenner's best friends ... TMZ has confirmed.

Our Jenner sources tell TMZ ... Bruce has hooked up with Ronda Kamihira ... a divorced, 51-year-old mother of 2.

We're told Kris is "extremely upset" that Bruce is dating Ronda, and -- as one source puts it -- "It really stings because Ronda never told Kris she started seeing Bruce."

And get this ... for years Ronda and the Jenners have been neighbors in Hidden Hills, CA, even spending holidays at each other's homes.

The daughters on both sides have become friends. Ronda's daughter, Spencer, took this pic with Kendall and posted it on her Twitter page.

A Jenner source tells us the talk within the family was that Ronda was weirdly becoming "Kris' single white female." The source says Ronda mimicked Kris right down to her hair.

This pic was taken at Staples Center in L.A., when Bruce and Ronda went to Elton John's concert.

And here's how awkward this is ... the pic (above) shows Ronda and Kris on vacation together in Dubai just a few years back.