Yes, as of Feb 3, 2016, the shows are part of the same fictional universe. (Note that the Arrowverse itself already includes a DC-comics-like incarnation of a multi-verse; it's highly likely that Kara Kent and Barry Allen will be from different internally-defined Earths, but part of the same externally-defined fictional reality.)
We have known for a while that this was a possibility, as the shows are produced by the same group of people, and CBS partially owns the CW. Originally the network execs were less enthusiastic to the idea, but when Supergirl's full-season pickup was announced, it was accompanied by rumors that a crossover might happen.
It has now been confirmed:
CBS announced today that Grant Gustin’s Flash will be crossing over into an episode of Supergirl later this year—a development that’s sure to please fans of both series. While details of what the crossover will entail are yet to be revealed, the episode is set to air on Monday, March 28th. src
Assuming there are no off-weeks, March 28th would be S01E20 of Supergirl, but no other details about that episode are known; the announcement also doesn't mention of the crossover will be reciprocal (e.g. there's no information that Kara Danvers will appear on the CW Arrowverse shows.) However, the description of the crossover as:
Grant Gustin for making the time to come visit
strongly implies that this one, at least, will be a one-way crossover.