The female of a new fossil genus and species, Rhombometridium pankowskiorum gen. and sp. nov. (Acari: Trochometridiidae), is described from Cretaceous Kachin amber. The new genus is characterized by the presence of well-developed sporothecae anteriad legs III, each sporotheca containing several ovoid fungal spores. This is the oldest (ca. 100 Mya), evidence of symbiosis between mites and fungi and the first fossil record of the mite family Trochometridiidae. The new genus shares synapomorphies with the family Trochometridiidae (presence of sporothecae between bases of legs III and IV) and Caraboacaridae (rhombic body shape, absence of setae v2, absence of palpal solenidion, palps fused with distinctly widened gnathosomal capsule), and potentially represents a transitional form or a missing link between these two closely related families. We also compare the sporothecae of R. pankowskiorum with those of other heterostigmatic mites.