Leptospirosis remains an important worldwide zoonotic disease caused by Leptospira spp affecting human and animals. This research aims to study the virulent-associated secreted proteins (protein secretome) of pathogenic Leptospira interrogans serovar Icterohemorrhagiae strain RGA (Leptospira RGA) transition from the environment to mammalian physiological osmolarity, temperature (37 °C) and carbon dioxide concentration (5% CO2) conditions for 24 h. Mass Spectrometry and bioinformatics approaches, we identified 69 potential secreted proteins from the culture supernatant of the Leptospira RGA isolate. We discovered transporters and porins such as phosphate porin, outer membrane efflux, ompA family protein, and polymer-forming cytoskeletal family protein under hyperosmotic condition. Under heat stress, degradation enzymes included zinc metallopeptidase, M23 family (LA3456, LA0709), Rhs family protein (LA1765), thermolysin metallopeptidase; / hydrolase family (LA1345, LA2501). Oxidative stress response proteins induced by osmolarity and temperature shifts included chaperon GrpE, DnaK (LA3705), antioxidants, i.e., thiol-specific redoxin, and peroxiredoxin (LA2809). In response to the in vivo transition, metabolic and other enzymes involved in energy production (COG:C), amino acid metabolism and transport (COG:E), and lipid metabolism and transport (COG:I), as well as moonlighting proteins functionally binding to plasminogen and fibronectin and regulating transcription, were also discovered. An overview of secreted proteins will supplement our understanding of Leptospira biology and pathogenesis during infection and also in response to environmental stimuli and their potential virulent determinants have the potential for developing leptospirosis vaccines and diagnosis.