The job of the Costume Designer is to decide on the clothes that all of the actors will be wearing on stage. The Costume Designer works with the Scenic Designer to create costumes that fit the actors and match the setting of the play. The Costume Designer reads the play, attends rehearsals, and looks at the Set Designer's design and research materials. The Costume Designer may also research the time period of the play to see what types of clothes were worn then. It is then the job of the Costume Designer to find the actual articles of clothing that the actors will wear. This can be done by making the costumes from scratch, using measurements and patterns from the actors' bodies or by buying or renting.

The costumes of Alexander… are modern variations of the clothes of the characters in Judith Viorst's 1972 book as illustrated by Ray Cruz. They reinforce the color scheme with the exception of certain stripe or waffle patterns as dictated by the book. For instance, Alexander's shirt throughout the play is like that in the book, of course given a blue waffle pattern. The illustrations of the book are black and white, so like the set, colors were guessed and applied to the costumes with the idea of creating a consistent theme.

In this play, clothes and costumes play a large role in the action and help to create the setting. Alexander's safari hat and the kangaroo and koala costumes transport the audience to Australia while all of the action in the shoe store revolves around very important, colorful costume pieces—shoes. For Alexander… the costumes are casual—very in tune with the fun, rugged clothes of kids. Most of the actors in the show wear t-shirts and jeans. The adults, of course, wear adult clothes—button-down shirts, slacks, skirts and blouses.

With a small cast, it was also very important to make the costumes versatile and easily changed or altered. It was imperative that Mom be able to put on glasses and a sweater to quickly become Mrs. Dickens, the teacher. It was similarly important that Dad be able to go from his work outfit to that of the shoe man. These changes must be done quickly and easily off stage with little help from other crewmembers or actors. Alexander is the only character who really changes his whole outfit—from pajamas to day clothes and back—and these changes are done easily, on stage behind the quilt being held up by actors. The costumes must be easy to take off and on with very little snapping, buttoning, or zipping.

During the tour, the costumes are carefully stowed with the rest of the show, sets and props, as they travel between theaters. The production crew handles their movement. The crew also does any alterations. Once initial design is done and the costumes all exist, either acquired or sewn, the designer's job is done.