In topology, a topological space is called **simply connected** (or **1-connected**, or **1-simply connected**^{[1]}) if it is path-connected and every path between two points can be continuously transformed (intuitively for embedded spaces, staying within the space) into any other such path while preserving the two endpoints in question. The fundamental group of a topological space is an indicator of the failure for the space to be simply connected: a path-connected topological space is simply connected if and only if its fundamental group is trivial.

A topological space is called *simply connected* if it is path-connected and any loop in defined by can be contracted to a point: there exists a continuous map such that restricted to is Here, and denotes the unit circle and closed unit disk in the Euclidean plane respectively.

An equivalent formulation is this: is simply connected if and only if it is path-connected, and whenever and are two paths (that is, continuous maps) with the same start and endpoint ( and ), then can be continuously deformed into while keeping both endpoints fixed. Explicitly, there exists a homotopy such that and

A topological space is simply connected if and only if is path-connected and the fundamental group of at each point is trivial, i.e. consists only of the identity element. Similarly, is simply connected if and only if for all points the set of morphisms in the fundamental groupoid of has only one element.^{[2]}

In complex analysis: an open subset is simply connected if and only if both and its complement in the Riemann sphere are connected. The set of complex numbers with imaginary part strictly greater than zero and less than one furnishes a nice example of an unbounded, connected, open subset of the plane whose complement is not connected. It is nevertheless simply connected. It might also be worth pointing out that a relaxation of the requirement that be connected leads to an interesting exploration of open subsets of the plane with connected extended complement. For example, a (not necessarily connected) open set has a connected extended complement exactly when each of its connected components are simply connected.

Informally, an object in our space is simply connected if it consists of one piece and does not have any "holes" that pass all the way through it. For example, neither a doughnut nor a coffee cup (with a handle) is simply connected, but a hollow rubber ball is simply connected. In two dimensions, a circle is not simply connected, but a disk and a line are. Spaces that are connected but not simply connected are called **non-simply connected** or **multiply connected**.

The definition rules out only handle-shaped holes. A sphere (or, equivalently, a rubber ball with a hollow center) is simply connected, because any loop on the surface of a sphere can contract to a point even though it has a "hole" in the hollow center. The stronger condition, that the object has no holes of *any* dimension, is called contractibility.

- The Euclidean plane is simply connected, but minus the origin is not. If then both and minus the origin are simply connected.
- Analogously: the
*n*-dimensional sphere is simply connected if and only if - Every convex subset of is simply connected.
- A torus, the (elliptic) cylinder, the Möbius strip, the projective plane and the Klein bottle are not simply connected.
- Every topological vector space is simply connected; this includes Banach spaces and Hilbert spaces.
- For the special orthogonal group is not simply connected and the special unitary group is simply connected.
- The one-point compactification of is not simply connected (even though is simply connected).
- The long line is simply connected, but its compactification, the extended long line is not (since it is not even path connected).

A surface (two-dimensional topological manifold) is simply connected if and only if it is connected and its genus (the number of *handles* of the surface) is 0.

A universal cover of any (suitable) space is a simply connected space which maps to via a covering map.

If and are homotopy equivalent and is simply connected, then so is

The image of a simply connected set under a continuous function need not be simply connected. Take for example the complex plane under the exponential map: the image is which is not simply connected.

The notion of simple connectedness is important in complex analysis because of the following facts:

- The Cauchy's integral theorem states that if is a simply connected open subset of the complex plane and is a holomorphic function, then has an antiderivative on and the value of every line integral in with integrand depends only on the end points and of the path, and can be computed as The integral thus does not depend on the particular path connecting and
- The Riemann mapping theorem states that any non-empty open simply connected subset of (except for itself) is conformally equivalent to the unit disk.

The notion of simple connectedness is also a crucial condition in the Poincaré conjecture.