This study examines the nonlinear effects of financial openness on financial development in the representative ASEAN member states: Indonesia, Malaysia, the Philippines, Singapore, and Thailand. It reveals that with a low (high) level of financial openness, the assets and liabilities of foreign direct investment increase (decrease) equity market capitalization and the financial development, institution, and market indices. We identify a threshold of financial openness where its positive impact begins to diminish; beyond this point, excessive financial openness may become detrimental to financial development. By reassessing the risks associated with financial openness after the Asian financial crisis, we can understand that the potential toxicity amplified by financial openness may pose threats to financial development.