Invasive sea lettuce (Ulva sp.) could be valorized in fish feeding insofar as protein content is increased. Direct NIRS measurement on fresh seaweed was tested as tool for monitoring protein enrichment of algae in different cultivation conditions. Spectra were collected on fresh spin-dried samples of algae with an ASD Labspec Pro spectrometer. The results of NIRS calibrations for dry matter (DM), total nitrogen (N) and total minerals (ash) on fresh samples led robust calibrations with SEP and R2 values of 1.08% and 0.93 for DM, 0.49%DM and 0.95 for N, 2.45%DM and 0.74 for ash. In order to compare calibration precision, calibrations were also developed on dried samples spectra collected on Foss NIRSYSTEM 6500 spectrometer. This study showed the interest of direct NIRS measurement as tool to daily monitoring the enrichment of algae (especially the protein content based on the N content) and its potential for decisions on harvest when N concentration reach a plateau.