A hallmark of wave-matter duality is the emergence of quantum-interference phenomena when an electronic transition follows different trajectories. This type of interference results in asymmetric absorption lines such as Fano resonances(1), and gives rise to secondary effects such as electromagnetically induced transparency when multiple optical transitions are pumped(2-5). Few solid-state systems show quantum interference and electromagnetically induced transparency(5-11), with quantum-well intersubband transitions in the infrared region(12,13) offering the most promising avenue to date to devices exploiting optical gain without inversion(14,15). Quantum interference is usually hampered by inhomogeneous broadening of electronic transitions, making it challenging to achieve in solids at visible wavelengths and elevated temperatures. However, disorder effects can be mitigated by raising the oscillator strength of atom-like electronic transitions-excitons-that arise in monolayers of transition-metal dichalcogenides(16,17). Quantum interference, probed by second-harmonic generation(18,19), emerges in monolayer WSe2, without a cavity, to split the frequency-doubled laser spectrum. The splitting exhibits spectral anticrossing behaviour, and is related to the number of Rabi flops the strongly driven system undergoes. The second-harmonic generation power-law exponent deviates strongly from the canonical value of 2, showing a Fano-like wavelength dependence that is retained at room temperature. The work opens opportunities in solid-state quantum-nonlinear optics for optical mixing, gain without inversion and quantum-information processing.