In nuclear chemistry and nuclear physics, J-couplings (also called spin-spin coupling or indirect dipole–dipole coupling) are mediated through chemical bonds connecting two spins. It is an indirect interaction between two nuclear spins that arises from hyperfine interactions between the nuclei and local electrons. In NMR spectroscopy, J-coupling contains information about relative bond distances and angles. Most importantly, J-coupling provides information on the connectivity of chemical bonds. It is responsible for the often complex splitting of resonance lines in the NMR spectra of fairly simple molecules.

J-coupling is a frequency difference that is not affected by the strength of the magnetic field, so is always stated in Hz.

The origin of J-coupling can be visualized by a vector model for a simple molecule such as hydrogen fluoride (HF). In HF, the two nuclei have spin ⁠1/2⁠. Four states are possible, depending on the relative alignment of the H and F nuclear spins with the external magnetic field. The selection rules of NMR spectroscopy dictate that ΔI = 1, which means that a given photon (in the radio frequency range) can affect ("flip") only one of the two nuclear spins. J-coupling provides three parameters: the multiplicity (the "number of lines"), the magnitude of the coupling (strong, medium, weak), and the sign of the coupling.

The multiplicity provides information on the number of centers coupled to the signal of interest, and their nuclear spin. For simple systems, as in ¹H–¹H coupling in NMR spectroscopy, the multiplicity is one more than the number of adjacent protons which are magnetically nonequivalent to the protons of interest. For ethanol, each methyl proton is coupled to the two methylene protons, so the methyl signal is a triplet, while each methylene proton is coupled to the three methyl protons, so the methylene signal is a quartet.

Nuclei with spins greater than ⁠1/2⁠, which are called quadrupolar, can give rise to greater splitting, although in many cases coupling to quadrupolar nuclei is not observed. Many elements consist of nuclei with nuclear spin and without. In these cases, the observed spectrum is the sum of spectra for each isotopomer. One of the great conveniences of NMR spectroscopy for organic molecules is that several important lighter spin ⁠1/2⁠ nuclei are either monoisotopic, e.g. ³¹P and ¹⁹F, or have very high natural abundance, e.g. ¹H. An additional convenience is that ¹²C and ¹⁶O have no nuclear spin so these nuclei, which are common in organic molecules, do not cause splitting patterns in NMR.

For ¹H–¹H coupling, the magnitude of J decreases rapidly with the number of bonds between the coupled nuclei, especially in saturated molecules. Generally speaking two-bond coupling (i.e. ¹H–C–¹H) is stronger than three-bond coupling (¹H–C–C–¹H). The magnitude of the coupling also provides information on the dihedral angles relating the coupling partners, as described by the Karplus equation for three-bond coupling constants.

For heteronuclear coupling, the magnitude of J is related to the nuclear magnetic moments of the coupling partners. ¹⁹F, with a high nuclear magnetic moment, gives rise to large coupling to protons. ¹⁰³Rh, with a very small nuclear magnetic moment, gives only small couplings to ¹H. To correct for the effect of the nuclear magnetic moment (or equivalently the gyromagnetic ratio γ), the "reduced coupling constant" K is often discussed, where

For coupling of a ¹³C nucleus and a directly bonded proton, the dominant term in the coupling constant J_C–H is the Fermi contact interaction, which is a measure of the s-character of the bond at the two nuclei.