Objective <p>To evaluate the impact of demand- and supply-side policies on the biosimilar market penetration and identify effective strategies for promoting biosimilar uptake in eight high-income countries. </p> Methods <p>We analyzed biosimilar market penetration for infliximab, rituximab, and trastuzumab in six European countries—France, Germany, Italy, Spain, Sweden, and the UK—and two Asian countries—Japan and South Korea. Biosimilar market penetration was measured using two indicators: biosimilar market share and the time required for biosimilars to reach the majority point (&gt;&#xa0;50% market share). Policies were categorized into demand- and supply-side measures, and weights were applied to reflect the extent and timing of policy implementation. Spearman correlation examined the relationship between policy implementation and biosimilar market penetration.</p> Results <p>Sweden, Italy, and the UK showed the highest biosimilar market shares, adopting various demand-side policies, while Japan and South Korea exhibited slower biosimilar adoption with fewer or no demand-side policies. Biosimilars in most European countries reached the majority point within 5–6 quarters, while projections for Japan and South Korea exceeded 30 quarters. Correlation analysis revealed that adoption of demand-side policies was significantly associated with higher market share (<i>r</i>_s&#xa0;=&#xa0;0.69, <i>p</i>&#xa0;&lt;&#xa0;0.001) and shorter time to reach the majority point (<i>r</i>_s&#xa0;=&#xa0;−&#xa0;0.62, <i>p</i>&#xa0;&lt;&#xa0;0.01). In contrast, supply-side policies showed a weaker and less consistent association.</p> Conclusions <p>Demand-side policies, such as financial incentives and prescribing guidelines, are significantly associated with rapid and widespread biosimilar adoption, while supply-side policies had limited impact. Policymakers should prioritize demand-side measures to improve biosimilar uptake and reduce healthcare expenditures effectively.</p>