We present a scanning tunneling microscope (STM) operating between 275 mK and room temperature in ultra-high vacuum (UHV) and in magnetic fields up to 14 T. The system features a compact STM fixed to a UHV compatible 3He insert fitting into a bottom loading cryostat with a superconducting magnet. Placing the cryostat on top of the UHV chamber allows a compact system design and an ideal thermalization of the STM, thus achieving a hold time of about 40 h at 275 mK. We show results obtained at 275 mK on a Bi2Sr2CaCu2O8+δ single crystal.