The underwater adhesion of marine mussels is a fascinating example of how proteinaceous adhesives, although water-soluble to begin with, can be used in seawater. Marine mussels adhere to the substrate via adhesive plaques, where the adhesive proteins are located especially at the substratum's interface. One major compound of the adhesives in Mytilidae is the mussel foot protein 3b (mfp-3b). Here, recombinant mfp-3b (rmfp-3b) was produced in Escherichia coli. rmfp-3b showed upper critical solution temperature (UCST) mediated complex coacervation at pH 3.0 in the presence of citrate yielding a liquid-liquid phase separation. Further, the rmfp-3b coacervation could also be induced in seawater conditions such as the respective pH and ionic strength, but without UCST behavior. In particular, sulfate and citrate anions could significantly induce complex coacervation. This study provides insights into the molecular behavior of one of the key proteins of mussels involved in underwater adhesion and may inspire new applications of bioadhesives using recombinant mussel foot proteins.