Czar Peter I of Bulgaria (927-969), the son of Czar Simeon the Great of Bulgaria, was married to Maria Irena, the granddaughter of Byzantine Emperor Romanus I Lecapenus. He ruled for over 40 years and was succeeded by their son Boris II.

Peter I tends to receive most of the blame for Bulgaria's decline. However, after the rule of Simeon, the peasantry, which constituted the main source of soldiers for the army, was depleted. The victories of Simeon the Great drained the nation's resources and resulted in very high taxes. Though often described as weak, sickly and meek, the quiet and modest Peter I remained on the throne longer than any other medieval Bulgarian ruler.

Under Peter I the wealthy and the higher clergy amassed more wealth, while the general population grew poorer. His own brothers organized plots against him. Losing the Serb lands early in his reign, he was unable to handle Bulgaria's external enemies, the Pechenegs repeatedly raided and plundered Dobrudja. Transylvania was lost and the Danube became the northern border of the state. To keep peace with the Byzantine Empire, Peter married the grand-daughter of Romanus Lecapenus and signed a thirty-year peace treaty under the acknowleged title of Tsar. Constantinople, however, kept up their attempts at thwarting the efforts of Bulgarian diplomacy.

Near the end of his reign, Boris and Roman, his sons, were taken hostage in Constantinople. Prince Svyatoslav of Kiev attacked Bulgaria's northeastern border, encouraged to do so by Emperor Nicephorus Phocas. Peter's army suffered a defeat in Dobrudja, and the Russians took eighty fortresses. Upon hearing the news Tsar Peter I suffered a stroke and took monastic vows. Soon after his death he was canonized by the Bulgarian Orthodox Church.