The retroviral psi packaging element, also known as the Ψ RNA packaging signal, is a cis-acting RNA element identified in the genomes of the retroviruses Human immunodeficiency virus (HIV)[1] and Simian immunodeficiency virus (SIV).

[2] It is involved in regulating the essential process of packaging the retroviral RNA genome into the viral capsid during replication.

In HIV, the psi element is around 80–150 nucleotides in length, and located at the 5' end of the genome just upstream of the gag initiation codon.

[8] It has a known secondary structure composed of four hairpins called SL1 to SL4 (SL is for Stem-loop) which are connected by relatively short linkers.

HIV-1 SL3 is sufficient by itself to induce heterologous RNA into virus-like particles but its deletion does not eliminate encapsidation.