Quinquagesima (/ˌkwɪŋkwəˈdʒɛsɪmə/), in the Western Christian Churches, is the last pre-Lenten Sunday, being the Sunday before Ash Wednesday, and the first day of Carnival (also known as Shrovetide). It is also called Quinquagesima Sunday, Quinquagesimae, Estomihi, Shrove Sunday, Pork Sunday, or the Sunday next before Lent.

Being the Lord's Day before the start of the Lenten season, it is known for meat consumption as people feast before starting their fast on Ash Wednesday, the first day of Lent. Historically Lutheran countries such as Denmark mark Quinquagesima Sunday as the peak of the Fastelavn. After attending the Divine Service on Shrove Sunday, congregants enjoy Shrovetide buns (fastelavnsboller). Children often dress up and collect money from people while singing. Christians in these nations carry Shrovetide rods (fastelavnsris), which "branches decorated with sweets, little presents, etc., that are used to decorate the home or give to children."

In the Revised Common Lectionary the Sunday before Lent is designated "Transfiguration Sunday", and the gospel reading is the story of the Transfiguration of Jesus from Matthew, Mark, or Luke. Some churches whose lectionaries derive from the Revised Common Lectionary, e.g. the Church of England, use these readings but do not designate the Sunday "Transfiguration Sunday".

The name Quinquagesima originates from Latin quinquagesimus (fiftieth). This is in reference to the fifty days before Easter Day using inclusive counting which counts both Sundays (normal counting would count only one of these). Since the forty days of Lent do not include Sundays, the first day of Lent, Ash Wednesday, succeeds Quinquagesima Sunday by only three days.

The name Estomihi is derived from the incipit or opening words of the Introit for the Sunday, Esto mihi in Deum protectorem, et in locum refugii, ut salvum me facias, ("Be Thou unto me a God, a Protector, and a place of refuge, to save me") Psalms 31:3

The earliest Quinquagesima Sunday can occur is February 1 and the latest is March 7.

In the Roman Catholic Church, the terms for this Sunday and the two immediately before it (Sexagesima and Septuagesima Sundays) were eliminated in the reforms following the Second Vatican Council, and these Sundays are part of Ordinary Time.

According to the reformed Roman Rite Roman Catholic calendar, this Sunday is now known by its number within Ordinary Time — fourth through ninth, depending upon the date of Easter. The earlier form of the Roman Rite, with its references to Quinquagesima Sunday, and to the Sexagesima and Septuagesima Sundays, continues to be observed in some communities.