The former Tottenham midfielder has signed for the Leicestershire outfit, who are in the FA Cup second round.
Harborough surprisingly beat Tonbridge Angels in the first round by a 4-1 margin.
The Southern League Premier Central club are in the seventh tier of the English pyramid.
Manager Mitch Austin said of signing Sandro: “What an unbelievable signing for the club, I really cannot express how I am feeling.
"I had a chance conversation with Sandro a couple of weeks ago and we got talking about football, told him about Harborough Town FC and the journey we are on and he has been messaging me ever since to sign on, totally unbelievable and gobsmacked is how I would describe this signing.
"These sort of signings don’t happen every day but when you get a chance to sign an ex-professional footballer who has played at the highest level in the Premier League and Internationally with Brazil, you just cannot turn it down."