<title>Abstract</title>Marine algae viruses are important for controlling microorganism communities in the marine ecosystem, and played a fundamental role during the early events of viral evolution. Here, we have focused on one major group of marine algae viruses, the ssDNA viruses from the <italic>Bacilladnaviridae</italic> family. We present the capsid structure of the bacilladnavirus, <italic>Chaetoceros tenuissimus</italic> DNA virus type II (CtenDNAV-II), determined at 2.3 Å resolution. Structural comparison to other viruses supports the previous theory where bacilladnaviruses were proposed to have acquired their capsid protein via horizontal gene transfer from a ssRNA virus. The capsid protein contains the widespread virus jelly-roll fold, but has additional unique features; a third β-sheet and a long C-terminal tail, which are located on the capsid surface and could be important for virus transmission. Further, low-resolution reconstructions of the CtenDNAV-II genome reveal a partially spooled structure, an arrangement previously only described for dsRNA and dsDNA viruses.