A twinless twin is a twin who has lost his or her sibling. This term is also used to describe other co-multiples, such as a triplet who has lost a sibling. Twinless twins are a topic of interest for some psychologists because their experiences are quite unique; even if the sibling dies in utero, a twinless twin often experiences psychological difficulties later in life which seem to be related to the loss of the co-multiple.

In some cases, a twinless twin loses a sibling in utero, due to complications of the pregnancy. Even though the twins never meet in life, their co-development in the womb apparently leaves a strong mark, and many twinless twins in this situation report feelings of loss or incompleteness later in life. A twinless twin may actually guess that he or she had a sibling in the womb before this fact is revealed, suggesting that a very strong connection is forged between co-multiples, even at an early gestational age.

One of the most common causes for a co-multiple to die during pregnancy is a condition known as vanishing twin syndrome. Vanishing twins die and are reabsorbed by the body, sometimes before the mother is even aware that she was carrying multiples. Estimates of the frequency of vanishing twin syndrome vary, but it may be as common as six out of every hundred pregnancies. The powerful bond between twins is different from that between other siblings, and it also endures beyond the confines of the uterus.

Because it is unusual for co-multiples to die simultaneously, every twin runs the risk of becoming a twinless twin. Twinless twins often experience immense psychological anguish, especially if their siblings lived into adulthood, allowing the twins to develop an even stronger bond. Twinless twins have reported a wide variety of symptoms, ranging from depression to fear of mirrors, and there are a number of twinless twin support organizations to connect twinless twins with people who understand what they are going through.

Studies seem to suggest that a twinless twin may sometimes feel obliged to live for two. Twinless twins may be driven, for example, to excel personally and professionally, to make up for the loss of a sibling's contribution to the world. Others have said that they feel obliged to work especially hard to please their parents. A twinless twin may also develop feelings of incompleteness, loss, or confusion as a result of the loss of the sibling, even if the two lived relatively independent lives.