Since its discovery, the laser (light amplification by stimulated emission of radiation) has spawned many innovations and become a ubiquitous technology. The maser (microwave amplification by simulated emission of radiation), on the other hand, whilst instrumental to the laser's birth and embodied in a variety of specialist devices, has not. The maser's relative obscurity to date has to a great extent been due to the severe, compounded encumbrances associated with all of its realizations: Atomic masers require vacuum chambers and pumps; as do free-electron masers. Solid-state masers, though excelling as low-noise amplifiers (incorporated within oscillators), have required refrigerators. Many realizations of either sort require (strong) magnets and/or magnetic shields. Freedom from these encumbrances -as is enjoyed by many lasers- would pave the way for new innovations (e.g. more sensitive electron paramagnetic resonance (EPR) spectrometers) exploiting the stunning sensitivity and explicit quantumness that maser amplification affords, yet in every-day settings. Here we report the first experimental demonstration of a solid-state maser operating at room temperature. It works on a bench, in air, in the earth's uncontrolled magnetic field, amplifying at around 1.45 GHz. In contrast to cryogenic ruby masers, its gain medium is an organic mixed molecular crystal, namely p-terphenyl doped with pentacene, that is photo-excited by yellow light. Its pumping mechanism exploits spin-selective molecular intersystem crossing (ISC) into pentacene's triplet ground state. When configured as an oscillator, its measured output power of around -10 dBm is approximately 100 million times greater than that of an atomic hydrogen maser, which -as it happens- oscillates nearby in frequency (near 1.42 GHz). By exploiting the milli-kelvin spin temperatures readily generated via ISC in photo-excited pentacene and/or other aromatic molecules, this new type of maser appears capable of amplifying at 100 times lower noise than the quietest known room-temperature microwave amplifiers.