After preseason trials, Ashby-Hammond has penned a 12-month contract with Argyle.
"He joined us in Austria, and it is not easy to come on a trip like that as a trialist, but he fit really well into the group," Plymouth head coach Tom Cleverley told the club website.
"He has a calm, focused character, impressed us on the training ground, and adds depth to our goalkeeping department."
Head of Football Operations David Fox added: “Luca showed us in his time on trial that he is a goalkeeper with a great deal of ability and, importantly, plenty of room for development.
"He has league experience, but is still young for a goalkeeper, and I know Darryl Flahavan is keen to work further with him. Luca has a great attitude to learn, and we’re excited with how we can help that.”