Monoclonal antibodies (mAb) used to treat cancers, immune and infectious diseases represent a multibillion dollar industry. Chinese Hamster Ovary cells are currently the gold standard. However, high expense, potential virus contamination and mAb heterogeneity represent major drawbacks that drive scientists to seek alternatives. Among these, the diatom P. tricornutum represent an option because of (i) its robustness in industrial process, (ii) its ability to be cultivated in mixotrophy, (iii) its genome availability and possibility to be genetically engineered. The proof of concept in P. tricornutum has already been exemplified through the production of a glycosylated mAb. A human-like N-glycosylation is now required for mAb optimal safety and efficacy when used in human therapies. Therefore, we propose to engineer the N-glycosylation pathway of P. tricornutum by gene silencing and expressing heterologous enzymes, thus allowing galactosylated algae-made mAb production.