| Conflict: | Battle of Thessalonica |
| Partof: | the Byzantine-Bulgarian Wars |
| Date: | 1040 |
| Place: | near Thessaloniki, Greece |
| Result: | Bulgarian victory |
| Combatant1: | Bulgarian Empire |
| Combatant2: | Byzantine Empire |
| Commander1: | Peter Delyan |
| Commander2: | Michael IV the Paphlagonian |
| Strength1: | Unknown |
| Strength2: | Unknown |
| Casualties1: | Unknown |
| Casualties2: | Unknown |
The battle of Thessalonica (Bulgarian: Битка при Солун) occurred in 1040 near the city of Thessalonica in contemporary Greece between the Bulgarians and the Byzantines. The battle ended with a Bulgarian victory.
In 1018 the Byzantines conquered Bulgaria after a bitter half-century struggle. In 1040 one Delyan who claimed to be a descendant of the Bulgarian Emperor Samuil led an uprising against the Byzantine occupation which broke out in Belgrade and was proclaimed Emperor under the name Peter II after the sainted Emperor Peter I (927-969). At the same time local Bulgarians in Dyrrhachium, in what is now Albania, revolted under the officer Tihomir. The two leaders met and to avoid distraction Peter Delyan was chosen as the only commander of the rebel army and Tihomir was killed.
After the two armies united Peter II Delyan marched eastwards to Thessalonica where at that time was the Byzantine Emperor Michael IV. The Byzantines were defeated and Michael IV had to flee for his life leaving his personal tent and large quantity of gold and silver.
Soon the Bulgarian successes continued as they seized the important Adriatic port Dyrrhachium. Another army invaded Thessaly and seized its northern parts. That caused serious troubles for the government in Constantinople which had to act swiftly in order to stop the restoration of the Bulgarian Empire.
The Byzantines gained a victory over the Bulgarians later in the year in the Battle of Thessalonica (2nd 1040).