The 24-year-old has signed a two-year contract after the two clubs agreed terms on a permanent transfer.
Gnanduillet scored four times in 17 appearances for Orient following a move to east London at the start of 2016.
"Armand brings height and power to the team and offers another option," boss Gary Bowyer told the club website.
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Blackpool have signed French striker Armand Gnanduillet from fellow League Two side Leyton Orient.