Lord Holmes of Richmond, 48, allegedly grabbed the woman's bottom after asking to touch her to see what she looked like - which the woman consented to.
He is also said to have asked if she did "extras" - to which the therapist told him she was a professional and tried to back away, a jury at Southwark Crown Court heard.
The 48-year-old allegedly asked: "Are you sure you've never done it?"
The incident is said to have happened near the end of a 90-minute deep tissue massage at a five-star hotel near parliament.
In his evidence, Lord Holmes said he had chosen the hotel due to its accessibility, adding that he often feels vulnerable in places that are difficult about allowing his guide dog.
The nine-time Paralympic swimming gold medallist has admitted asking to touch the masseuse but denies any sexual motivations.
The court heard he had asked to touch a different beauty therapist's face in the same hotel, but that she refused.
"What does it matter what the massage therapist looks like?" she told him.
Lord Holmes said: "I wasn't asking to touch her to see if she was male or female, tall, short, fat or thin - it's just to get the sense of another person and see what she looks like.
"I understand how difficult it is to imagine what it must be like to have no sight whatsoever, because up until I was 14 I couldn't have any indication of what that would be like, but that really is my world.
"My world would stop here (in front of me) if I couldn't contact that external world that you can get in the blink of an eye, and I try and use everything I've still got to try and construct that world.
"So, through sound, smells, and, yes, touch, but touch as a means of being able to construct that world, touching objects, and, yes, touching people every single day."
He said it allowed him to "get a sense of that other person - not to make a facsimile or an oil painting of them, just having a sense of that other person who was in a room that I didn't really know... lying on my back and feeling completely vulnerable."
The former athlete, who retired from sport in 2002 and became a member of the House of Lords in 2013, denies one count of sexual assault.
The trial continues.