NGC1313 X-2 was among the first ultraluminous X-ray sources discovered, and has been a frequent target of X-ray and optical observations. Using the HST/ACS multi-band observations, this source is identified with a unique counterpart within an error circle of 02. The counterpart is a blue star on the edge of a young cluster of <10^7 years amid a dominant old stellar population. Its spectral energy distribution is consistent with that for a Z=0.004 star with 8.5 M_ about 5×10^6 years old, or for

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... n O7 V star at solar metallicity. The counterpart exhibited significant variability of Δ m = 0.153±0.033 mag between two F555W observations separated by three months, reminiscent of the ellipsoidal variability due to the orbital motion of this ULX binary.