(1) Background: Due to the risk factors involved and the special characteristics of the game, rugby players are reported to have elevated levels of injury. The aim of this longitudinal prospective cohort study with a 2b level of evidence is to investigate the incidence of injury in the Spanish Men’s Senior National Rugby XV team, identify the most frequent injury types, and the game actions that cause them. (2) Methods: we studied a total of 72 players from the Spanish Men’s Senior National Rugby XV team while they played the Nation’s Cup for four consecutive seasons. We recorded all the injuries occurring both during games and training sessions, according to World Rugby consensus. (3) Results: Out of a total of 27 injuries, the higher incidence was during the matches as opposed to the training sessions (100.0 [95% CI: 62.0–132.9] per 1000 player hours (ph) and 1.17 [95% CI: 0–2.5] per 1000 ph during matches and training, respectively). The most common injuries were muscular injuries (41.6 [95% CI: 16.3–66.9] per 1000 ph) and concussion (29.1 [95% CI: 7.8–50.4] per 1000 ph). Tackling caused more injuries (50.0 [95% CI: 22.4–77.5] per 1000 ph). (4) Conclusions: Muscular injuries and concussion are the most common, 28% of concussion cases are diagnosed after the match, and tackling is the game action causing most of these injuries, with a higher incidence in players who play the whole match.