The present invention relates to a biologically active single chain Relaxin peptide having the following formula (I): Nter-Ac-X10-E-G-R-E-X15-V-R-X18-X19-I-X21-X22-E-G-X25-S-X27-X28-X29-X30-X31-X32-X33-NH2-Cter or a salt or solvate thereof. It also concerns a pharmaceutical composition comprising at least one peptide of the invention, and the peptide or the pharmaceutical composition for its use as a medicament.