This study analyzes the mathematical knowledge to teach early algebra exhibited by pre-service early childhood education teachers, from the perspective of the Mathematical Knowledge for Teaching (MKT) model. The research adopts a mixed exploratory-descriptive methodological approach, based on the application of the MKT-early algebra questionnaire (3–6), consisting of six open-ended items that place teachers in various teaching situations reflecting the knowledge that characterizes early algebra at this stage of schooling. The analysis of the answers given by the pre-service teachers of early childhood education revealed a level general of insufficient mathematical knowledge, with the common content knowledge exhibiting fewer limitations compared to the other subdomains that comprise the model, and the horizon content knowledge the weakest. We conclude that it is necessary to offer teacher training programs that deepen the didactics of early algebra and provide tools to further the effective teaching of this content block in early childhood education.