Callowhill (also styled CallowHill Guitars) is a Philadelphia-based custom bass builder founded by luthier Tim Cloonan. A graduate of Temple University's jazz performance program who began on guitar before shifting to lutherie, Cloonan built each instrument to order at the Callowhill shop until his death in October 2016. The brand grew out of an ongoing R&D dialogue with working session and touring bassists, and became known for lightweight, hand-built instruments aimed at professional players.

The flagship MDM is offered in 5- and 6-string configurations and is typically built around a swamp ash body, a three-piece maple neck with wenge center strips, Nordstrand pickups, and a passive wiring stage with a modified Aguilar OBP-3 preamp. Standard hardware includes Hipshot Ultralite tuners and a Hipshot 'B' style bridge, with silver Teflon-coated internal wiring. The catalog also includes the Junk, Infini, and MPB5, alongside one-off commissions such as the MonoNeon Microtonal Bass — a fretted instrument with a quarter-tone fretboard layout.

Callowhill basses have been played by bassists including Tim Lefebvre, Owen Biddle of The Roots, Derrick Hodge, Steve Jenkins, and MonoNeon (Dywane Thomas Jr.).

Callowhill has 2 products cataloged on Equipboard, including Electric Basses. Their gear is featured by 2 artists, with the strongest followings in Rock, Electronic, and Pop. Notable users include Tim Lefebvre and MonoNeon.