Tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3OH, is a long-expected new mineral of the tourmaline supergroup. It occurs in an aplitic dike of a LCT-type pegmatite body from Grotta d’Oggi, San Pietro in Campo, island of Elba, Italy, in association with quartz, K-feldspar, plagioclase, elbaite, and schorl. Crystals are greenish yellow with a vitreous luster, a white streak, and show no fluorescence. Tsilaisite has a Mohs hardness of approximately 7; it is brittle with a sub-conchoidal fracture, and has a calculated density of 3.133 g/cm3. In plane-polarized light, tsilaisite is pleochroic, O = pale greenish yellow, E = very pale greenish yellow; it is uniaxial negative, ω = 1.645(5), ɛ = 1.625(5). Tsilaisite is rhombohedral, space group R3m, a = 15.9461(5), c = 7.1380(3) Å, V = 1571.9(1) Å3, Z = 3. The strongest eight X-ray-diffraction lines in the powder pattern [d in Å(I)(hkl)] are: 3.974(100)(220), 2.942(94)(122), 2.570(79)(051), 2.034(49)(152), 4.205(41)(211), 6.329(22)(101), 2.377(21)(003), and 1.592(21)(550). Analysis by a combination of electron microprobe, secondary ion mass spectrometry, and optical absorption spectroscopy gives SiO2 = 36.10(3), TiO2 = 0.32(4), Al2O3 = 37.10(5), MnO = 9.60(10), CaO = 0.09(4), Na2O = 2.11(7), K2O = 0.03(1), F = 0.79(3), B2O3 = 10.2(6), Li2O = 0.8(1), H2O = 3.1(2), sum 99.95 wt%. The unit formula is X(Na0.67□0.30Ca0.02K0.01)Y(Mn2+1.34Al1.14Li0.54Ti0.04)ZAl6T(Si5.94Al0.06)B2.91O27V(OH)3W(OH0.39F0.41O0.20). The structure, refined also taking into account the positional disorder of the O1 and O2 anions, converged to statistical indices R1 for all reflections of about 2%. The resulting site populations indicate that the Z site is occupied by Al and that the Y site is dominated by Mn2+. Aluminum is incorporated at Y through two types of substitutions: YAl+WO2− → YMn2++WOH, which has the result of replacing OH at the W site by O2−, and Y(Al+Li)+WF → 2YMn2++WOH, which relates fluor-elbaite to the tsilaisite component. Infrared absorption spectra measured in the principal OH-stretching region were interpreted on the basis of local arrangements consistent with the short-range bond-valence requirements. A compositional trend from fluor-elbaite to tsilaisite is observed in samples from Elba Island. The occurrence of tsilaisite is very rare in nature, as a consequence of both the requirement of extraordinary petrogenetic conditions and limited structural stability.