In 2008, Blecher and Labuschagne extended Beurling's classical theorem to H ∞invariant subspaces of L p (M, τ ) for a finite von Neumann algebra M with a finite, faithful, normal tracial state τ when 1 ≤ p ≤ ∞. In this paper, using Arveson's non-commutative Hardy space H ∞ in relation to a von Neumann algebra M with a semifinite, faithful, normal tracial weight τ , we prove a Beurling-Blecher-Labuschagne theorem for H ∞ -invariant spaces of L p (M, τ ) when 0 < p ≤ ∞. The proof of the main result relies on proofs of density theorems for L p (M, τ ) and semifinite versions of several other known theorems from the finite case. Using the main result, we are able to completely characterize all H ∞ -invariant subspaces of L p (M⋊ α Z, τ ), where M ⋊ α Z is a crossed product of a semifinite von Neumann algebra M by the integer group Z and H ∞ is a non-selfadjoint crossed product of M by Z + . As an example, we characterize all H ∞