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The heavily obscured open cluster Berkeley~51 shows characteristics typicalof young massive clusters, even though the few previous studies have suggestedolder ages. We combine optical ($UBV$) and 2MASS photometry of the clusterfield with multi-object and long-slit optical spectroscopy for a large sampleof stars. We apply classical photometric analysis techniques to determine thereddening to the cluster, and then derive cluster parameters via isochronefitting. We find a large population of B-type stars, with a main sequenceturn-off at B3$\,$V, as well as a large number of supergiants with spectraltypes ranging from F to M. We use intermediate resolution spectra of theevolved cool stars to derive their stellar parameters and find an essentiallysolar iron abundance. Under the plausible assumption that our photometryreaches stars still close to the ZAMS, the cluster is located at$d\approx5.5\:$kpc and has an age of $\sim60\:$Ma, though a slightly youngerand more distant cluster cannot be ruled out. Despite the apparent good fit ofisochrones, evolved stars seem to reside in positions of the CMD far away fromthe locations where stellar tracks predict Helium burning to occur. Ofparticular interest is the presence of four yellow supergiants, two on theascending branch and two others close to or inside the instability strip.

Berkeley 51, a young open cluster with four yellow supergiants