Selenocystine is the amino acid with the formula (HO2CCH(NH2)CH2Se)2.

It is the oxidized derivative of the canonical amino acid selenocysteine (HO2CCH(NH2)CH2SeH).

The compound can also be prepared synthetically from serine.

[2] Because selenocysteine is not easily isolated or handled, it is often generated by reduction of selenocystine in situ.

[3] The selenium–selenium bond length is 2.321 Å, which is 14% longer than the disulfide bond in cystine at 2.040 Å.