20 of those reserve grade matches came in 2023 with the Roosters where he was able to construct an impressive campaign.
A confirmed departure at the recently-held Roosters' end of season awards night, the Giants have now announced he will join for the 2024 and 2025 seasons.
Deakin is an English-born player, originally from Oldham, and could slot straight into a significant role with the Giants following the retirement of Nathan Peats.
Deakin said the lure of playing top-tier footy was behind the decision.
"Funnily enough, I went to watch Ian Watson's team play against St Helens in the Grand Final a few years back," he said in a statement.
"I was watching them and following them and they did something special. So when I heard that Huddersfield wanted me and that he was the head of that project its an opportunity I jumped at.
"I think it's the next step in my career, I have been at the Rosters since under 13's and it's a big part of my life, but I think it's time to start playing some first team footy."
Deakin, who played in pre-season trials for the Roosters, has been on Huddersfield's radar for much of the year.
"Thomas is a young British nine, who we've kept a close eye on ever since he featured in the trial games for Sydney during pre-season," the club's head coach Ian Watson said.
"He's played in the Reserve game competition for Sydney during 2023, and we've been watching how he's performed.
"I feel that Thomas will be a great fit for the organisation, he's also a boxing champion so he comes with the right work ethic and mentality and that's the sort of player we want to promote through the Giants.
"He's been well educated and in a great environment in the Roosters system, while also training with the first team during pre-season, he'll a learnt so much from the players they have there.
"Attack wise he's quick from dummy-half and has some real good distribution, defensively he's not afraid of putting his body in and can also fit in at loose if needed, giving us good strength in our spine."