We present results of recent Neutron Star Interior Composition Explorer observations of the accreting millisecond X-ray pulsar IGR J17062 - 6143 that show that it resides in a circular , ultracompact binary with a 38 minute orbital period . NICER observed the source for \approx 26 ksec over a 5.3 day span in 2017 August , and again for 14 and 11 ksec in 2017 October and November , respectively . A power spectral analysis of the August exposure confirms the previous detection of pulsations at 163.656 Hz in Rossi X-ray Timing Explorer data , and reveals phase modulation due to orbital motion of the neutron star . A coherent search for the orbital solution using the Z ^ { 2 } method finds a best-fitting circular orbit with a period of 2278.21 s ( 37.97 min ) , a projected semi-major axis of 0.00390 lt-sec , and a barycentric pulsar frequency of 163.6561105 Hz . This is currently the shortest known orbital period for an AMXP . The mass function is 9.12 \times 10 ^ { -8 } M _ { \odot } , presently the smallest known for a stellar binary . The minimum donor mass ranges from \approx 0.005 - 0.007 M _ { \odot } , for a neutron star mass from 1.2 - 2 M _ { \odot } . Assuming mass transfer is driven by gravitational radiation , we find donor mass and binary inclination bounds of 0.0175 - 0.0155 M _ { \odot } and 19 ^ { \circ } < i < 27.5 ^ { \circ } , where the lower and upper bounds correspond to 1.4 and 2 M _ { \odot } neutron stars , respectively . Folding the data accounting for the orbital modulation reveals a sinusoidal profile with fractional amplitude 2.04 \pm 0.11 \% ( 0.3 - 3.2 keV ) .