The 23-year-old completed her third spell at Sydney with an appearance for the club on Christmas Eve but could be pitched straight into action for the struggling Foxes when the WSL resumes with Leicester's home game against Brighton & Hove Albion at King Power Stadium on Sunday.
Siemsen told Leicester's official website on Thursday: "To be joining such a prestigious club is exciting for myself and my family.
"We've all grown up watching English football, and being able to come across and play with a club like Leicester City is a dream come true.
"Everyone has been so welcoming and I feel right at home already."
Born in St Leonards, Siemsen, who began her career with Manly United, is no stranger to overseas stints, having played in the US for with California Storm and in Sweden with AIK.
This, though, promises to be her biggest challenge, with Leicester having yet to pick up a single point this season in the English top-flight league and destined for relegation unless she can help fashion a fightback over the second half of the season.
Goals have been Leicester's biggest problem, with only two having being notched in their nine defeats so far.
Leicester replaced manager Lydia Bedford with former Everton boss Willie Kirk in November but their fortunes have not improved, with Sunday's game against second-from-bottom Brighton, who have seven points, looking a must-win.
Much will be expected of Siemsen, who made her debut for Australia against Brazil in October 2021 and has won five caps in all.
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