The 24-year-old, who can play at flyhalf or centre, would join Fumiaki Tanaka (Highlanders) and Shota Horie (Melbourne Rebels) in the southern hemisphere competition once his Kubota Spears team completed its Top League campaign, Kyodo news agency reported.
Wallabies great and Kubota Spears coach Toutai Kefu has declared Tatekawa, capped 20 times by the Brave Blossoms, as "the best Japanese player I've seen".
"How that translates to Super Rugby is a bit unknown," he told Canberra Times. "I think he can do it, he's a talented kid. Skill-wise he can do everything and he's a big, tough boy."
The former number eight's 1999 World Cup winning team mate and Brumbies coach Stephen Larkham appeared convinced of Tatekawa's credentials.
"Another player in that 10-12 position is great for us, it allows depth and we're quite lucky to have Haru," Larkham told the newspaper which said Tatekawa's arrival in Canberra was a part of a development plan.
"There's going to be a lot of competition from those positions and Haru will definitely be pushing for one of those spots given his form for the Japanese team."
"The standard of footy in Japan is a little bit less than Super Rugby, but I think individual players have the skills and when you put them in a good system you'll see improvement," he added.
The Brumbies enjoyed a successful 2013 Super Rugby campaign, winning the Australian conference and two tough playoff matches before going on to lose 27-22 to the Waikato Chiefs in the final.