In mathematics, infinity plus one is a concept which has a well-defined formal meaning in some number systems, and may refer to:

• Transfinite numbers, numbers that are larger than all the finite numbers.
  • Cardinal numbers, representations of sizes (cardinalities) of abstract sets, which may be infinite.
  • Ordinal numbers, representations of order types of well-ordered sets, which may also be infinite.
• Hyperreal numbers, an extension of the real number system that contains infinite and infinitesimal numbers.
• Surreal numbers, another extension of the real numbers, contain the hyperreal and all the transfinite ordinal numbers.