The 29-year-old joined the Knights late in the 2023 pre-season, making 12 NRL appearances after debuting for the club in the opening round against the New Zealand Warriors in Wellington.
Miller will join the Leeds Rhinos at the start of next season with the star fullback also able to fill in at half-back or on the wing.
He featured for Australia at Tokyo 2020 in the men's rugby union sevens team who reached the quarter-finals.
"I am looking forward to joining such a prestigious club as Leeds Rhinos and excited to come across having spoken with Rohan (coach) and Gary to be part of the culture at Headingley," Miller told the club's official website.
"I really enjoyed my footie at the start of this season and I want to bring that to the Rhinos. It is a big change for my family, we have a newborn and two-year-old so it is a once-in-a-lifetime opportunity for our young family that we are ready to embrace and make the most of."
Head coach Rohan Smith said: "Having spoken to people about Lachie, everyone speaks about him being well respected and highly regarded by those who have worked with him.
"He is a quick, broken-field runner who is gaining in experience all the time having come across to league from rugby union later in his career.
"I think he will fit in well with our squad for next season and the plans we have for our style of play moving forward."
The Knights have embarked on a seven-match winning run as they prepare for finals football under coach Adam O'Brien.
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