The Hydra spectrum is similar to that of Pluto's largest moon, Charon, which is also dominated by crystalline water ice.
But Hydra's water-ice absorption bands are even deeper than Charon's, suggesting that ice grains on Hydra's surface are larger or reflect more light at certain angles than the grains on Charon.
Pluto's outermost small moon, Hydra is thought to have formed in an icy debris disk some four billion years ago.
Mission scientists are investigating why Hydra's ice seems to be cleaner than Charon's.
"Perhaps micrometeorite impacts continually refresh the surface of Hydra by blasting off contaminants," said Simon Porter, New Horizons science team member from Southwest Research Institute in Boulder, Colorado.
"This process would have been ineffective on the much larger Charon, whose much stronger gravity retains any debris created by these impacts," he added in a Nasa statement.
The New Horizons science team is looking forward to obtaining similar spectra of Pluto's other small moons, for comparison to Hydra and Charon.
The new data was gathered by New Horizons from a distance of 240,000 km on July 14 last year.