Phosphoserine (abbreviated as SEP or J) is an ester of serine and phosphoric acid.

Phosphoserine is a component of many proteins as the result of posttranslational modifications.

[1] The phosphorylation of the alcohol functional group in serine to produce phosphoserine is catalyzed by various types of kinases.

[2][3] Through the use of technologies that utilize an expanded genetic code, phosphoserine can also be incorporated into proteins during translation.

[7] Phosphoserine has three potential coordination sites (carboxyl, amine and phosphate group) Determination of the mode of coordination between phosphorylated ligands and metal ions occurring in an organism is a first step to explain the function of the phosphoserine in bioinorganic processes.