Ljubomir Obradovic admitted carrying out the offence, which took place in the jacuzzi at a health club in August 2016.
The pensioner admitted having his victim sit astride him in the hot tub while he touched her legs.
Chris Smyth, defending, said his client, of Church Farm Cottages, Easton Grey, near Malmesbury, had no previous convictions.
He said the behaviour he is admitting is what he admitted to when he was questioned by police, as the prosecution were not relying on what the victim told them.
Obradovic, he said, had worked as a farm manager since the 1970s and is now retired.
He said that he lives on a cottage on the farm rent free for the rest of his life as result of his service to his former employer.
Judge Robert Pawson adjourned the case to February 8 so a pre-sentence report can be prepared and released him on conditional bail.
The pensioner was told he must not have contact with the victim or go to the health suite at the Corsham Community Campus.
He must also register as a sex offender, the total time he must continue to do so will be determined by the sentence he receives.
The judge said “The fact that I am directing a pre-sentence report should not be taken by you as any indication of what the sentence will be: all sentencing options are open to the court.”