Two undescribed Plasmodium species were present in 1 of 8 New Guinea skinks, Tribolonotus gracilis. Plasmodium tribolonoti n. sp. is characterized by rounded or oblong schizonts 6.1 × 5.3 μm (5–7 × 4–7) that produce on average 14.3 merozoites (10–21), in no particular arrangement of nuclei. All parasitized proerythrocytes and had no effect upon dimensions of host cells or their nuclei. Gametocytes, mostly erythrocytic, averaged 7.2 × 6.3 (6.5–9.0 × 5.5–7.5), with length × width (LW) 45.5 μm² (38–63) and L/W 1.15 (1.0–1.5). Gametocytes are not sexually dimorphic in size or shape and had no effect upon dimensions of host erythrocytes or their nuclei. Schizonts of P. gracilis n. sp. averaged 4.3 × 3.5 (3–6 × 3–5), with 4.9 merozoites (3–8) usually arranged as a fan, and had no effect upon dimensions of host erythrocytes or their nuclei. Gametocytes averaged 5.9 × 5.5 (5.0–6.6 × 5.0–6.0), with LW 31.9 μm² (25–40) and L/W 1.06 (1.0–1.2). Gametocytes are not sexually dimorphic in size or shape and had no effect upon dimensions of host erythrocytes or their nuclei.