The dataset provided here consists of sighting data of sea turtles (Caretta caretta) collected between April 2013 and October 2017, along the ferry route between Italy and Spain, within the Fixed Line Transect Mediterranean monitoring Network (FLT Med Net), a multidisciplinary monitoring project coordinated by ISPRA since 2007. The programme uses ferry boats as observation platforms to undertake surveys of marine macro-fauna and main threats (maritime traffic and floating marine litter), and involves several International partners that share common protocols for data collection across different routes in the Mediterranean Sea. This datasets includes 117 georeferenced sightings of loggerhead turtles, together with effort data covered in good weather conditions (Beaufort <4), for a total of approximately 79.700 km, over 333 surveyed transects.