The phytohormone auxin affects numerous processes in land plants. The central auxin signaling machinery, called the nuclear auxin pathway, is mediated by its pivotal receptor named TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX (TIR1/AFB). The nuclear auxin pathway is widely conserved in land plants, but auxin also accumulates in various algae. Although auxin affects the growth of several algae, the components that mediate auxin signaling have not been identified. We previously reported that exogenous auxin suppresses cell proliferation in theKlebsormidium nitensthat is a member of streptophyte algae, a paraphyletic group sharing the common ancestor with land plants. AlthoughK. nitenslacks TIR1/AFB, auxin affects the expression of numerous genes. Thus, elucidation of the mechanism of auxin-inducible gene expression inK. nitenswould provide important insights into the evolution of auxin signaling. Here, we show that some motifs are enriched in the promoter sequences of auxin-inducible genes inK. nitens. We also found that the transcription factor KnRAV activates several auxin-inducible genes and directly binds the promoter ofKnLBD1, a representative auxin-inducible gene. We propose that KnRAV has the potential to regulate auxin-responsive gene expression inK. nitens.