The naval battles of Massilia (49 BC) were two victories won by Caesar's naval commander Decimus Brutus during the siege of the same town.
Massilia (modern Marseille) had chosen to side with Pompey and the Senate (Great Roman Civil War), accepting Lucius Domitius Ahenobarbus as governor of Transalpine Gaul and commander of the city during the siege. This was a major problem for Caesar, who had decided to move against Pompey's supporters in Spain. He laid siege to Massilia with three legions, before leaving Caius Trebonius in command and heading to Spain. Twelve warships were built at Arelate (or Arelas, modern Arles) and placed under the command of Decimus Brutus.
The Massiliotes also possessed a fleet, reported by Caesar as consisting of eleven decked ships of war, six un-decked ships of war and a number of smaller ships. These ships were manned by local sailors, and large numbers of archers and warriors from the nearby Albiei tribe.
The Massiliote fleet came out to attack Brutus's smaller squadron, which was based at an island opposite the city. The two fleets fought in different ways. The Massiliotes relied on the speed and agility of their ships, hoping to surround Caesar's fleet and carry out massed attacks on individual ships.
Brutus relied on the skill of his legionaries and hoped to board the enemy ships. These tactics paid off. Caesar's men were happy to be attacked by two enemy ships at once, grappling with them both and sending boarding parties onto both. The Massiliotes were forced to retreat back into harbour, having lost nine ships, some sunk and some captured. Caesar doesn't say what type of ships these nine were.
The second battle took place after Cneius Pompey sent a fleet of sixteen ships under Lucius Nasidius to aid the defenders. Once he reached the vicinity of Massilia he sent messengers into the city to ask the Massiliotes to risk another battle. They restored their fleet to its original size by repairing older ships found in the port, then sailed to Taurois, where they met up with Nasidius. The Massiliotes made up the right division of the combined fleet, Nasidius the left division.
Brutus also had a larger fleet for this second battle, having added six of the captured ships to his original twelve. Confident after his earlier victory he led his fleet towards Taurois.
This time the Massiliotes appear to have held their own for some time, even coming close to trapping Brutus between two of their three-decked galleys. Brutus managed to escape from this trap, and the two enemy ships were damaged in a collision caused by their eagerness to get into the fight. Some of Brutus's nearby ships spotted their discomfort and both were sunk.
The turning point appears to have come when Nasidius fled from the scene of the battle, taking one of the Massiliote ships with him. Of the Massiliote ships one managed to return to the city, five were sunk, four captured and the rest escaped to Spain.
These two naval victories allowed Trebonius to continue with the siege, which only ended in victory after Caesar's return from Spain.