We present the design and implementation of a medium-band near-IR filter tailored for detecting low-mass stars and brown dwarfs from the summit of Maunakea. The W-band filter is centered at 1.45 micron with a bandpass width of 6%, designed to measure the depth of the H2O water absorption prominent in objects with spectral types of M6 and later. When combined with standard J- and H-band photometry, the W-band filter is designed to determine spectral types to $approx$1.4 subtypes for late-M and L dwarfs, largely independent of surface gravity and reddening. This filters primary application is completing the census of young substellar objects in star-forming regions, using W-band selection to greatly reduce contamination by reddened background stars that impede broad-band imaging surveys. We deployed the filter on the UH 88-inch telescope to survey $approx$3 sq. deg. of the NGC 1333, IC 348, and rho Ophiuchus star-forming regions. Our spectroscopic followup of W-band selected candidates resulted in the confirmation of 48 ultracool dwarfs with a success rate of 89%, demonstrating the efficacy of this new filter and selection method.
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