PC Daniel Moss, 39, who was based in Hastings, was placed under formal investigation by Sussex Police on 2 December and suspended from duty.
The force said he was advertising sexual services for payment on an adult website after reporting off sick with a stress-related illness in September.
He was sacked on Friday following a public gross misconduct hearing.
The hearing found he had behaved in a manner that had discredited the police service or undermined public confidence.
He did not attend the hearing.
Det Insp Nick Wainwright from Sussex Police's professional standards department said despite being on sick leave Mr Moss was "still able to offer himself as being available to perform sexual services for payment".
Mr Wainwright said it was "clearly completely out of keeping with his role that others uphold with pride, integrity and with the trust of the public whom they serve".

Ambulance staff dealt with 2,184 calls between 19:00 on Hogmanay and 07:00 on New Year's Day.
Additional crews, call handlers and dispatchers were on duty. The busiest time was from 01:00 to 04:00, with 739 calls.
The new year total was 35 higher than the previous year. The Christmas period, from 23 to 27 December, saw a 10% increase on 2015.
Scottish Ambulance Service chief executive Pauline Howie said: "The overall level of demand for our service over the festive period has exceeded previous years and Hogmanay was one of our busiest nights of the year."

Stephen Dodd tweeted an image of solicitors Asif Bodi and Abubakar Bhula on prayer mats in a stairwell.
He captioned the tweet: "Muslims praying at half time at the match yesterday. Disgrace."
The Premier League club reported the tweet to Merseyside Police following a number of complaints.
A number of social media users shared the photograph, taken during Liverpool's FA Cup quarter-final against Blackburn on 8 March.
In a statement it said police had now referred the issue back to the club, which was "currently in the process of determining the appropriate action to take".
It added:  "Liverpool Football Club would like to remind its supporters that it does not tolerate any form of discrimination and is committed to ensuring that the club and Anfield provides a welcoming and safe environment for all fans regardless of race, religion, gender, age, disability or sexual orientation."
The club said a further statement would be made in due course.
Speaking last month, Mr Bodi, 46, from Preston, told the Liverpool Echo: "We have a small window in which to pray, a bit like the transfer window in football. Once that closes the chance has gone.
"That day, the time came for prayer and the window would have closed before the game finished so we did it at half-time.
"Most people are absolutely fine with it...but it only takes one or two people to react in the wrong way and you could have a nasty situation."

Emergency services were called to the fire in Insch just after midnight when a local resident noticed smoke emerging from a property in High Street.
The Scottish Fire and Rescue Service said the blaze had breached the roof of the building.
Residents in nearby flats were evacuated to a community centre but there were no casualties.
Six fire engines from nearby towns, including a height appliance from Aberdeen, were sent to tackle the fire.
Crews, including four firefighters wearing breathing apparatus, used water jets to douse the flames.
The fire service said the blaze was finally extinguished at 02:23 but they remained on the scene for several hours.
They added that an investigation will be carried out in due course.
A police officer who advertised himself as a male prostitute while off sick from work has been sacked.
[NEXT_CONCEPT]
The Scottish Ambulance Service had a busier Hogmanay than last year.
[NEXT_CONCEPT]
Liverpool FC is considering what action to take against a supporter who branded two Muslim fans "a disgrace" for praying during half-time at a match.
[NEXT_CONCEPT]
Residents were evacuated from their homes in an Aberdeenshire town after a blaze broke out in a nearby bank.