Sainfoin (Onobrychis spp.) has been cultivated as a fodder crop in the Near East since ancient times (Aygün et al. 2018). Given this, Onobrychis peloponnesiaca may be viewed as a wild relative of Sainfoin under Taxon group 4, according to the taxon group definition by Maxted et al. (2006). The species was originally described as a subspecies of O. aliacmonia Rech.f. (Tan and Iatrou 1996) but has since been elevated to species status (Greuter & Raus (ed.) 1999).

Onobrychis peloponnesiaca is endemic to the Peloponnese region of Greece, occupying a highly restricted distribution between the villages of Vlachiotis and Molaoi. The extent of occurrence and area of occupancy for the species are both estimated to measure 12 km². The total population size is estimated to number no more than 150 mature individuals, with no more than 50 in the largest subpopulation. The major threats to the species are associated with the destruction of remaining habitat for agricultural expansion and infrastructure development. As agricultural expansion or road maintenance may represent the greatest threat to particular individuals, the species is estimated to be restricted to two locations. Extensive cultivation is apparent across its distribution, and road grading and clearance of vegetation resulted in the loss of two thirds of the population between 2003 and 2023, though it is not known how much of this reduction occurred within the last one generation length of the species. The species is not known to occur within protected areas, and thus future conversion of habitat for agriculture or infrastructure development represents a highly plausible future threat. All individuals are suspected to occur within close proximity to the Vlachiotis-Molaoi road and agricultural land. Given this, it is inferred that road maintenance activities and cultivation practices, such as the application of herbicides, are contributing to an ongoing decline in the quality of habitat and the number of mature individuals. On account of its restricted distribution, population, and ongoing declines, this species is assessed as Critically Endangered under criterion C2a(i). Given the restricted distribution and population size of this species, ongoing monitoring of population trends and threats is required, alongside a more thorough evaluation of current threatening processes. As the species is not known to occur within any protected areas, and its survival is considered to rely upon protection of its habitat, site protection and habitat restoration represent essential conservation measures. It is possible that the species may coexist alongside traditional olive cultivation; accordingly, education of landowners about the threatened status of this species is also recommended.

Onobrychis peloponnesiaca is endemic to the Peloponnese region in Greece. The only known subpopulations occur in the prefecture of Laconia between the villages of Vlachiotis and Molaoi (Kalpoutzakis and Constantinidis 2009). The species has been recorded between 100–200 m alt. (Tan and Iatrou 1996, Kalpoutzakis and Constantinidis 2009).

The population of the species was estimated to comprise a total of 200 mature individuals in 2009. Near the fork in the road leading to Apidea, around 50 plants have been counted. Suitable habitat is highly limited as a result of extensive cultivation (Kalpoutzakis and Constantinidis 2009). Near Vlachiotis, 150 individuals were observed in 2002. This number increased to 250 individuals following clearance of habitat to establish olive groves; however, road widening resulted in the loss of individuals such that the subpopulation size had fallen back to 150 individuals by 2009. According to Constantinidis (pers. comm. 2023), the road widening and the creation of a waste dump site has now diminished the subpopulation to only 50 individuals or fewer. He stated that both subpopulations are so easily susceptible to human activities and so vulnerable to any change, such as establishment of new crops, pesticides, garbage disposal, pollution, deforestation, etc., that they could easily be destroyed.

This species is a small perennial herb with several erect-ascending stems growing up to 0.75 m in height (Tan and Iatrou 1996). It occurs in stony areas with shrubs and phrygana, in sunny or rarely semi-shady spots, on marls, on the outskirts of cultivated areas and olive groves, and on hills with maquis vegetation. Near the village Vlahiotis it appears on low hills that remain uncultivated, along with other Greek endemics, such as Ebenus sibthorpii, Astragalus laconicus and Thymus laconicus, at 100-150 m altitudes. Near the village Apidea it grows at 160-200 m atlitude, on marls, in sparse maquis vegetation, where Quercus coccifera, Pistacia lentiscus and Calicotome villosa are dominant. Flowering occurs between April and May (Kalpoutzakis and Constantinidis 2009).

The major threats to this species are associated with the destruction of remaining habitat for agricultural expansion and infrastructure development. Road grading and clearance of vegetation resulted in the loss of two thirds of the population's largest subpopulation, and the remaining individuals are now more susceptible to future road maintenance activities (Constantinidis pers. comm. 2023). Extensive cultivation is apparent across its distribution, and the species is not known to occur within a protected area (UNEP-WCMC and IUCN 2020), thus further agricultural expansion is a plausible and serious threat. Where the species occurs within, or adjacent to, cultivated areas, it is suspected that it is threatened by cultivation practices, such as the eradication of weeds and use of herbicides. However, unlike many other species in the genus, O. peloponnesiaca is not known to be grazed by livestock (Kalpoutzakis and Constantinidis 2009). In order to successfully mitigate against these threats, further research is required to better evaluate their scale and severity.

Onobrychis peloponnesiaca is a wild relative of Sainfoin under Taxon Group 4, according to the taxon group definition by Maxted et al. (2006). Research is recommended to determine whether the species represents an appropriate candidate for use in gene transfer to the crop. It should be noted that unlike many other species in the genus, O. peloponnesiaca is not known to be grazed by livestock (Kalpoutzakis and Constantinidis 2009).

One Onobrychis peloponnesiaca germplasm accession of wild origin, collected in Greece, is held by the Millennium Seed Bank Project in the UK (Millennium Seed Bank Partnership Data Warehouse 2022). The species is represented in living collections at two botanic gardens (BGCI 2023). The smaller known subpopulation occurs close to the boundaries of the Ori Anatolikis Lakonias Special Protection Area (GR 2540007). The species was listed as Endangered in The Red Data Book of Rare and Threatened Plants of Greece. Its survival relies upon the protection of its remaining habitat, and the prevention of further conversion of habitat to agricultural land use or transport networks. It is possible that the species may coexist alongside traditional olive cultivation, and therefore education and incentivisation of landowners represent other essential conservation measures (Kalpoutzakis and Constantinidis 2009). Agricultural practices and road maintenance activities are inferred to be contributing to an ongoing decline in habitat quality. Therefore, in situ as well as ex situ conservation measures are mandatory. Currently, the taxon is found in two Botanical Gardens (BCGI 2023).