  We report the discovery of the likely orbital period of the ultracompact
low-mass X-ray binary (LMXB) 2S 0918-549. Using time-resolved optical
photometry carried out with the 8-m Gemini South Telescope, we obtained a
2.4-hr long, Sloan r' light curve of 2S 0918-549 and found a periodic,
sinusoidal modulation at 17.4+/-0.1 min with a semiamplitude of 0.015+/-0.002
mag, which we identify as the binary period. In addition to 4U 0513-40 in the
globular cluster NGC 1851 and the Galactic disk source 4U 1543-624, 2S 0918-549
is the third member of the ultracompact LMXBs that have orbital periods around
18 min. Our result verifies the suggestion of 2S 0918-549 as an ultracompact
binary based on its X-ray and optical spectroscopic properties. Given that the
donor in 2S 0918-549 has been suggested to be either a C-O or He white dwarf,
its likely mass and radius are around 0.024--0.029 M_sun and 0.03--0.032 R_sun,
respectively, for the former case and 0.034--0.039 M_sun and 0.033--0.035 R_sun
for the latter case. If the optical modulation arises from X-ray heating of the
mass donor, its sinusoidal shape suggests that the binary has a low inclination
angle, probably around 10 deg.
