Background Cadherin‐associated protein p120 catenin regulates cell adhesion and migration in cell cultures and is required for axial elongation in embryos. Its roles in adhesion and cell migration are regulated by phosphorylation. We determined the effects of phosphorylation of six serine and three threonine residues in p120 catenin during zebrafish (Danio rerio) embryogenesis. Results We knocked down endogenous p120 catenin‐δ1 with an antisense RNA‐splice‐site morpholino (Sp‐MO) causing defects in axis elongation. These defects were rescued by co‐injections of mRNAs for wildtype mouse p120 catenin‐δ1‐3A or various mutated forms. Several mRNAs containing serine or threonine codons singly or doubly mutated to phosphomimetic glutamic acid rescued, and some nonphosphorylatable mutants did not. Conclusions We discovered that phosphorylation of serine residue S252 or S879 is required for convergent extension of zebrafish embryos, since rescue occurred only when these residues were mutated to glutamic acid. In addition, the phosphorylation of either S268 or S269 is required, not both, consistent with the presence of only a single one of these residues in two isoforms of zebrafish and Xenopus laevis. In summary, phosphorylation of multiple serine and threonine residues of p120 catenin activates migration of presomitic mesoderm of zebrafish embryos facilitating elongation of the dorsal axis. Key Findings Knockdown of zebrafish p120 catenin‐δ1 with a morpholino blocks migration of presomitic mesoderm and extension of the dorsal axis. We compared rescue of morphology by injecting mouse p120 catenin‐δ1‐3A (CTNND1‐3A) mRNAs mutated to phosphomimetic (E) and nonphosphorylatable (F) versions of serine and threonine residues. Convergent extension of zebrafish embryos was partially rescued by phosphorylation of T310, S905, or T916. Importantly, it required phosphorylation of serine residues S252, S879, and either of S268 or S269. We see normal morphogenesis as a dynamic interplay of motile filopodia and lamellipodia forming at the leading edge of the presomitic mesoderm cell as well as turnover of new cadherin‐mediated anchoring adherens junctions that facilitate a treadmilling action used by presomitic cells to migrate toward the dorsal aspect of the embryo.