Escherichia phage T7 is the type species of the Autographivirales, an order of bacteriophages that all encode an RNA polymerase (RNAP). Here, we characterize the RNAP derived from Pseudomonas phage Njord and investigate its utility for in vitro transcription (IVT). Njord belongs to a subfamily of phages found in marine environments and its RNAP displays high transcriptional activity at temperatures of 25 °C and below. Properties of Njord RNAP relevant for the synthesis of therapeutic mRNA are presented, with attention given to promoter specificity, abortive transcription, fidelity, RNA 5′ and 3′ homogeneity, incorporation of cap analogs and modified nucleotides, and double-stranded RNA (dsRNA) byproduct formation. Comparative analysis of RNA produced by Njord or T7 RNAP highlights a specific origin of immunostimulatory dsRNA and demonstrates that Njord produces less dsRNA during IVT. As a result, luciferase mRNA made with Njord RNAP elicits a minimal immune response when transfected in A549 cell culture or when introduced into mice as a lipid nanoparticle. Njord RNAP emerges as a cold-adapted homolog of T7 RNAP with advantages for the synthesis of certain therapeutic mRNA modalities.