The main aim of the present study is to measure the capability the 'Posidonia Oceanica' as fibre reinforcement for adobe bricks. This sea-plant is distributed in the Mediterranean coast, considered as a solid marine by-product. To analyse the performance, prismatic earthen specimens with seagrass fibres were compared with the most traditional additives for this purpose; straw fibres. Both fibres were included with different lengths and quantities. Previously, in order to understand their behaviour, the fibres themselves were evaluated. Tensile strength and water absorption tests were performed. Meanwhile bricks themselves were subjected to mechanical tests. Concerning the biomass fibres, results show that straw fibres present higher tensile resistance than the seagrass leafs while they are more fragile to breakage than seagrass; on the other hand, they have higher water absorption than seagrass. Mechanical results show variations depending on their fibre content and length. Nevertheless, bricks with long seagrass fibres present a characteristic good behaviour in terms of flexural and compressive strength. Generally, adobe bricks with both types of fibres achieve similar mechanical properties, generating favourable results in terms of comparison between them. The work presented was partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER)). The authors of this paper would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537). GREA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 642384 Buildings as Material Banks (BAMB).