The Love Actually star endorsed the Labour candidate for West Bromwich East after the pair campaigned against Rupert Murdoch's media empire and called for tighter regulation of the Press.
But Mr Grant's endorsement comes at the same time as he is preparing to host a dinner for the Liberal Democrats in a bid to boost their tiny campaign coffers.
The junior party in the coalition has seen its spending dwarfed by union backed Labour, business backed Conservatives and even UKIP, which enjoyed a £1.3 million donation from newspaper mogul Richard Desmond. Mr Grant said: "I'm not a Labour person, or a Tory or anything else, but four years campaigning with the Hacked Off campaign has taught me that the more important dividing line in Parliament is between the members with real principle and guts and those without.
"Of the (smaller) good group, there's none better than Tom Watson. I wish he could be our next Prime Minister to be honest. He's Black Country, bright and ballsy. Love from Hugh Grant. "
Mr Watson said: "I'm deeply flattered to receive such kind words from Hugh Grant but the endorsement that matters most to me is that of local people in West Bromwich.
"We've had a tough but fair campaign in West Bromwich East and I'd like to thank all my political opponents for making the campaign about issues not personalities. Whatever party local people support I hope they will use their vote to make a difference on Thursday."
Mr Grant's endorsement of Mr Watson is along similar lines to that which he has given the Liberal Democrats. People who donated money to the Lib Dems before tonight were in with a chance of sitting down to eat with Mr Grant.
In an email sent out by former leader Paddy Ashdown, Mr Grant said: "I am not a Lib Dem, a Tory a Labourite or anything in particular but I recognise political guts."