Andrew Harvey, 42, was working at St Ann's Hospital in Poole supervising a woman who was being treated for depression and a personality disorder.
Dorset Police said he encouraged her to book into a hotel and engaged in consensual sexual activity.
He was jailed for 21 months for offences under the Sexual Offences Act 2003.
At an earlier hearing he pleaded guilty to three counts of sexual activity with a person with a mental disorder by a care worker.
Bournemouth Crown Court heard the offences happened in 2015 when Harvey, of Chaddesley Wood Road in Poole, worked for the NHS as a mental health support worker on a bank nurse basis.
The woman had been on a supervised trip to see her children when they arrived early and Harvey encouraged her to book a hotel room.
Det Con Ben Griffin, of Bournemouth CID, said: "While the victim in this case consented to having sex with Andrew Harvey, he knew what he was doing was wrong and abused his position of trust with a vulnerable woman."
A spokesman for Dorset Healthcare University NHS Trust said: "We take any abuse of patients very seriously.  It was the trust that reported this incident to the police and we acted immediately to ensure Mr Harvey had no access to patients."
A mental health support worker has been jailed after admitting sexual activity with a vulnerable patient.