Self-splicing is one of the methods that may be used to remove introns from a pre-mRNA molecule to form the mature mRNA transcript. It is a post-transcriptional modification. Self-splicing occurs on the rare occasion when an intron forms a ribozyme (RNA enzyme) that is capable of catalysing its own excision from the molecule, in the same manner that its excision is usually catalysed by a spliceosome complex.

There are three groups of self-splicing introns, though groups I and II require no assistance from proteins.

In group I introns, two transesterifications lead to the excision of introns and splicing of exons:
  1. The 3' hydroxyl (OH) group of a free guanine nucleoside (guanosine), found either within the intron or elsewhere, attacks the phosphate at the 5' end of the splice site
  2. The 3' hydroxyl (OH) group of the 5' exon joins (splices) with the 3' exon

In group II introns, there are also two transesterifications:
  1. The 2' OH of a specific adenosine nucleoside in the intron attacks the 5' splice site, forming a lariat
  2. The 3' OH of the 5' exon triggers the next transesterification, at the 3' splice site, splicing the two exons together