• A dependently-typed language that features implicit polymorphism, general recursion and explicit type-level computation. • Higher ranked subtype reasoning about the more-general-than relation between polymorphic types. • Fully mechanized proof of the type soundness and transitivity. A polymorphic subtyping relation, which relates more general types to more specific ones, is at the core of many modern functional languages. As those languages start moving towards dependently typed programming a natural question is how can polymorphic subtyping be adapted to such settings. This paper presents the dependent implicitly polymorphic calculus ( λ I ∀ ): a simple dependently typed calculus with polymorphic subtyping. The subtyping relation in λ I ∀ generalizes the well-known polymorphic subtyping relation by Odersky and Läufer (1996). Because λ I ∀ is dependently typed, integrating subtyping in the calculus is non-trivial. To overcome many of the issues arising from integrating subtyping with dependent types, the calculus employs unified subtyping , which is a technique that unifies typing and subtyping into a single relation. Moreover, λ I ∀ employs explicit casts instead of a conversion rule, allowing unrestricted recursion to be naturally supported. We prove various non-trivial results, including type soundness and transitivity of unified subtyping. λ I ∀ and all corresponding proofs are mechanized in the Coq theorem prover.