Background: Raise in platelet count called thrombocytosis, which is mostly an incidental laboratory finding may be primary (essential thrombocytosis) or secondary (reactive thrombocytosis). The reactive thrombocytosis can be caused due to malignant or non-malignant hematological conditions; acute or chronic inflammatory conditions and tissue damage. This study, was aimed to analyze the clinical spectrum of reactive thrombocytosis, grade thrombocytosis according to the etiologies and analyze the distribution of platelet indices in various clinical settings of secondary (reactive) thrombocytosis. Objectives: The objective of this study was to analyze various clinical patterns and etiologies causing reactive thrombocytosis, to grade reactive thrombocytosis based on various clinical settings and etiologies, to analyze the distribution of platelet indices in various clinical conditions and to analyze the associated peripheral smear findings observed in various clinical settings of secondary thrombocytosis. Methodology: An observational study done among the patients attending the OPD of General Medicine Department of Trichy SRM medical college for a period of 6 months. All patients found to have thrombocytosis (platelet count >4.5lakhs/mm3) were included, while those with primary thrombocytosis after complete clinical evaluation and corelating with the laboratory parameters, known case of myeloproliferative disorders and those who did not consent for the study were excluded. Thus, a total of 289 samples, were studied to identify the clinical spectrum, grade of thrombocytosis and analyze the distribution of platelet indices in various clinical settings of secondary(reactive) thrombocytosis. Results: The mean age of the study participants was found to be 40.16±20.34 years (range 1 to 83 years). 11.4% of who had reactive thrombocytosis were children <10 years. Reactive thrombocytosis was more prevalent among those in 3rd and 4th decade of life, as 36.3% belonged to this age group and 19.1% more than 60 years. The major cause of reactive thrombocytosis was infection (52.3%), inflammation (14.2%), iron deficiency anemia (13.8%) and 15.9% being idiopathic. Around 75% of the reactive thrombocytosis were mild and only 4.2% and 1.7% had severe and extreme reactive thrombocytosis. Majority (47.5%) of iron deficiency anemia had giant platelets compared to infection (47.02%) who mainly had small platelets. While 75.61% of the patients with inflammation had small platelets and 58.70% of idiopathic conditions presented with small platelets. All the patients who had thrombocytosis due to the effect of drugs had giant platelets. Conclusion: Though the reactive thrombocytosis per se doesn’t result vascular and hemostatic complications, the underlying cause should be found and treated. The grading of thrombocytosis serves a good purpose in analyzing the severity of infection, inflammation and malignant conditions thus facilitating in adequate modification of management techniques.