This study describes non-invasive photoacoustic imaging to detect and monitor the growth of conjunctival melanomas in vivo. Conjunctival melanomas were induced by injection of melanotic B16F10 cells into the subconjunctival space in syngeneic albino C57BL/6 mice. Non-invasive in vivo photoacoustic tomography was performed before, and after tumor induction up to 2 weeks. Spectral unmixing was performed to determine the location and to assess the distribution of melanin. The melanin photoacoustic signal intensity was quantified from the tumor-bearing and control eyes at all timepoints. For postmortem validation, total tumor and melanotic tumor volumes were measured using H&E stained tumor sections and were compared to in vivo photoacoustic imaging measurements. Photoacoustic imaging non-invasively detected eyes bearing conjunctival tumors of varying sizes. The melanin signal was detected as early as immediately following injection of melanotic tumor cells. Changes in tumor size over time were assessed with changes in the volume and intensity of the melanin signal. Four growing tumors and one regressing tumor were observed. Three tumors without significant change in signal intensity over time were observed, showing variable growth. Photoacoustic melanin signal on the last day of in vivo imaging correlated with postmortem total tumor volume (R2 = 0.81) and melanotic tumor volume (R2 = 0.80). The results of our study show that actively growing conjunctival melanomas can be quantified in a non-invasive manner using in vivo photoacoustic tomography. The photoacoustic melanin signal intensity correlated with total and melanotic tumor volume. This novel in vivo imaging platform may help to assess new treatment modalities to manage ocular tumors.