1 Serpentis (1 Ser) is a red giant in the constellation Virgo with an apparent magnitude of 5.5. It is a red clump giant, a cool horizontal branch star that is fusing helium in its core. It has expanded to over 13 times the radius of the Sun and although it is cooler at 4,581 K it is 77 times more luminous. It is 322 light years away.

The Flamsteed designation 1 Serpentis was given to the star when the constellation Serpens was combined with the constellation Ophiuchus. It was also given the Bayer designation M Serpentis. When Ophiuchus and Serpens were separated into distinct constellations, 1 Serpentis was left over the border in Libra. Since then it has moved slightly and is now in Virgo.

A 10th-magnitude companion star discovered by William Herschel is 86″ away. It is at the same distance as 1 Ser and shares a common proper motion, It is considered likely to be a physical companion, with the two stars separated by 8,600 au. It has a spectral type of G5 IV, and it is slightly smaller and less luminous than the sun.

A much more widely-separated 10th-magnitude star is also listed in multiple star catalogues, but it is an unrelated background object.