The tweet was subsequently deleted, with the force later saying it had been "taken out of context".
The message was intended to form part of PSNI's "without consent" campaign, launched in 2013, which has previously run into criticism for allegedly "victim blaming" over a tweet in March warning women about the dangers of "overindulging".
One Twitter user, @69mib, wrote: "Bumping into someone, under the mistletoe, is now considered rape? Why do you trivialise such a serious issue with such a nonsense tweet?".
Another, Mike Love, responded: "This tweet tells people that a kiss is rape. Do you realise what you've said?"
While the offence of sexual assault involves intentionally sexually touching another without consent and without reasonably believing the other consents, rape must involve penetration with a penis.
PSNI later appeared to defend the thrust of its earlier message, tweeting on Sunday: "We posted a message on Twitter yesterday that some may have taken out of context but the message remains the same; when you are out socialising over the Christmas period, please remember without consent it is rape."
Some Twitter users appeared more offended about what the messages said about the police force's attitude to Christmas than crime.
One responded: "Don't put a downer on things sure! Very cheerful Mr PSNI"; other people said they were astounded that @PoliceServiceNI was not a spoof account.
In August a probe was launched following allegations that six PSNI officers and staff had tweeted sexist and racist messages, as well as those that could aggravate sectarian messages, which may potentially prejudice criminal investigations.
Some of the tweets were said to have included allegations against a Northern Irish journalist and also the abuse of another journalist from a Belfast radio station.
Kissing under the mistletoe is a centuries-old tradition inspired by a belief that the plant increases life and fertility.
Historical accounts suggest the ancient Norse tradition was to kiss the hand of a host under the mistletoe, rather than the cheek or lips.
This article originally appeared on the Daily Telegraph.