The open access book set LNCS 16556 + 16557 constitutes the refereed proceedings of the 27th International Symposium on Formal Methods, FM 2026, held in Tokyo, Japan, during May 18–22, 2026. The 49 full and 2 short papers included in these proceedings were carefully reviewed and selected from 239 submissions. The proceedings also include 4 invited talks, 8 tutorial papers, and 9 papers from the industry day track. The contributions were organized in topical sections as follows: Part I: Invited Papers; and Main Track (1) Part II: Main Track (2); Tests and Proofs Track (TAP); Tutorial Track; and Industry Day Track