Isaach Isaachsen (25 May 1774 – 15 August 1828) was a Norwegian politician.
He was born in 1774 as the son of wealthy businessman Daniel Isaachsen and his first wife Christiane Hedvig Christensen.
He married a Hedevig Elisabeth Hansen.
He was the brother of Peder and Daniel Otto Isaachsen.His son Daniel became a jurist.
Through him, Isaach Isaachsen was the grandfather of painter Olaf Isaachsen.He was elected to the Norwegian Parliament in 1818, representing the constituency of Lister og Mandals Amt.