The agar extraction process of the red algae Gelidium sesquipedale generates a solid residue as the main by-product. However, this residue remains non-valorized, despite its potential as a fertilizer. This study aims to determine the value of G. sesquipedale residue as organic fertilizer and for soil amendments. An incubation test of G. sesquipedale residue in soils was performed to measure the nitrogen (N), phosphorus (P), and calcium (Ca) release. The potential fertilization effect of the residue was evaluated in a greenhouse on two crops: strawberry and corn. The amount of available P was high at the beginning of the incubation experiment. The amounts of nitrate–nitrogen (NO_₃⁻-N) and available Ca increased over the incubation time. A high efficiency of fertilization using the residue at different concentrations was observed in both crops. Application of the residue enhanced crop growth. The fertilization effect was associated with increased macro- and micro-elements in the strawberry fruit’s N, Ca, iron (Fe), manganese (Mn), and zinc (Zn) and in the corn leaves’ N, P, magnesium (Mg), and Fe. Moreover, the residue was a good soil organic amendment as it enhanced the amount of organic matter (OM) and some macro- and micro-elements in the soil after plant harvest.