A pentaquark is a hypothetical subatomic particle consisting of four quarks and one antiquark bound together (compared to three quarks in normal baryons). As quarks have a baryon number of +¹⁄₃, and antiquarks of −¹⁄₃, it would have a total baryon number of 1, thus being classified as an exotic baryon. The existence of pentaquarks was originally hypothesized by Michał Praszałowicz in 1987.

Several experiments reported the existence of pentaquark states in the mid 2000s, but later experiments and re-analysis of the data showed them to be statistical effects rather than true resonances.

In the mid 2000s, several experiments claimed to reveal pentaquark states. In particular, a resonance with a mass of 1,540 MeV/c² (4.6 σ) was reported by LEPS in 2003, the Θ+
. This coincided with a pentaquark state with a mass of 1,530 MeV/c² predicted in 1997 by Dmitri Diakonov, Victor Petrov, and Maxim Polyakov.

This state was presumably composed of two up quarks, two down quarks, and one strange antiquark (uudds). Following this announcement, nine other independent experiments reported seeing narrow peaks from nK+
and pK0
, with masses between 1,522 MeV/c² and 1,555 MeV/c², all above 4 σ. While concerns existed about the validity of these states, the Particle Data Group gave the Θ+
a 3-star rating (out of 4) in the 2004 Review of Particle Physics. Two other pentaquark states were reported albeit with low statistical significance—the Φ−−
(ddssu), with a mass of 1,860 MeV/c² and the Θ0
c (uuddc), with a mass of 3,099 MeV/c². Both were later found to be statistical effects rather than true resonances.