Officers from the Garda Siochana Ombudsman Commission were today preparing a file for the Director of Public Prosecutions after spending several hours questioning the man yesterday.
The garda, in his 20s and based in a station in the Midlands, met members of the investigation team by appointment at Loughrea station in Co Galway yesterday morning.
It is alleged the serious sexual assault took place over a year ago.
It is believed the alleged victim is an eastern European prostitute who was previously investigated by the gardai.
This was the first time since the Ombudsman Commission was set up five years ago that they have arrested a rank and file officer.
His arrest was made under section 4 of the Criminal Justice Act, which meant he could have been held without charge for up to 24 hours
It is understood he was arrested after he had earlier declined, on legal advice, to co-operate with the commission's inquiries.
The officer has been suspended by the garda authorities pending the outcome of the commission's inquiries.
The woman has already been interviewed.