Walkes has made just one appearance for the Spurs first team - which came in the EFL Cup in September 2016 - but the 20-year-old recently returned from a loan spell at MLS side Atlanta United.
Manager Kenny Jackett has now brought in three youngsters during January - with Sylvain Deslandes and Connor Ronan both arriving from Wolverhampton Wanderers - but the Welshman is confident that Walkes can add something different to his first-team squad.
The 56-year-old told the club's official website: "He can play in midfield, the centre of defence or at right-back and he's an athletic player, so we're pleased to add him for the rest of the season.
"The experience of coming to a club like Pompey will be a good one for him and he'll be keen to help us move towards the top six. He's someone I have a lot of time for and think he's got a very bright future. He's certainly got all the tools to succeed."
Walkes scored twice in 20 appearances during his time with Atlanta in the MLS.