The Stedelijk's jarring addition strikes discord in Amsterdam's cultural enclave.

For nine years, the Stedelijk Museum has been Amsterdam's most forlorn and hopeful institution—shuttered, vacant, and in terrible need of an overhaul that was always just about to get started and would surely be finished sometime soon. Finally, all the delays have been overcome, overruns absorbed, and embarrassments set aside: The Netherlands' modern-art mecca can finally reopen. Its collection, which stretches from van Gogh to last week's wunderkind via Mondrian, Malevich, Picasso, de Kooning, and Warhol, is returning to public view. Unfortunately, the museum that contains such bounty has disappeared.

The flamboyant neo-Renaissance palazzo from 1895 hasn't vaporized, of course. Seen from the old front entrance—now the rear—its red-brick facade, striped with white stone bands and topped by fanciful Dutch gables and pyramidal turrets, looks cleaner and more exuberant than ever. But approach it from the bustling greensward of the Museumplein—or Museum Plaza—and the original building appears to be cowering behind a glossy white bathtub. This outsized plumbing fixture, clad in para-aramid, a synthetic-fiber panel, and finished with airplane paint to give it that enameled luxury look, is the long-awaited, crushingly disappointing extension designed by the Amsterdam-based firm Benthem Crouwel.

Something about this project brings out the worst in major talents. First Robert Venturi and then Álvaro Siza got the job, and both came up with unworkable designs. Now long deliberation and careful planning have produced a new wing that doubles the institution's bulk, blares its presence, and solves some internal problems while creating a whole passel of new ones for a troubled public space.

There's much to like on the inside. Visitors will have a more focused, better-lit, smoother, and more properly climate-controlled experience than they ever could before. The new Stedelijk can once again start borrowing works from other institutions, which had been loath to expose their fragile treasures to its damp and drafts. The addition touches the original building only gently, sealing the old rear facade inside the new glass entrance lobby. A double-height escalator threads through a bright yellow tube and down to a 10,000-square-foot, column-free underground gallery for really big art.

Lead architect Mels Crouwel has preserved the beloved white-brick interiors of the original, and at the same time ensured that old and new galleries flow together in a smoothly continuous sequence. The blind arcade and crown moldings remain in place at the top of the grand stair (now the school-group entrance); elsewhere, unadorned white walls float a few inches off the floor. Gone is the familiar herringbone parquet, replaced by pale wood flooring that would be the pride of a Swedish sauna.

There's a conceit at work here: The seamless integration of the old and new interiors correlates with the seamless sheen of the new building's envelope. In theory, the defiant juxtaposition in architectural styles highlights the continuity of the artistic experience. But this reasoning seems awfully gaseous when you're confronted with the way the building asserts itself on the city.

The museum borders the Museumplein, a vast open plaza where for more than a century Amsterdammers have gathered to ice-skate, listen to concerts, protest, or just loll in the sun. The square has had a tortuous evolution. At the end of the 19th century, as Amsterdam expanded, city fathers envisioned not just a national museum—the Rijksmuseum—but an entire cultural district, organized around an ample park. Vienna was opening its Ringstrasse, Paris its Haussmannian boulevards, and New York was erecting a massive Metropolitan Museum with an 800-acre backyard called Central Park. Amsterdam built three cultural châteaux: the Rijks- and Stedelijk museums and the Concertgebouw, home of the storied royal orchestra. The great green mall that stretched between these national homes of art and music had to negotiate a change in orientation, since the Concertgebouw turned its facade coquettishly aside to conform to a new street plan. Perhaps it's that slight kink that has allowed successive generations to nibble at the park's borders and muddy its symmetries.

The Museumplein has been designed, destroyed, redesigned, and tinkered with, blurring the gracious original almost beyond recognition. Invading German armies filled it with bunkers. Postwar planners ran a multilane highway through it. In 1973 the Van Gogh Museum expanded into the park with Gerrit Rietveld's severe brick block, then did so again in 1999, with an appalling addition by Kisho Kurokawa. In the 1990s the landscape architect Sven-Ingvar Andersson had tried to clean up the mess. He replaced part of the roadway with a reflecting pool and greened over the rest—but he also sliced up the square with light paths that killed off symmetry once and for all. Andersson's boldest move was the ezelsoor (donkey's ear), created by peeling up one corner of the lawn above a buried supermarket. On one side is an attractive and popular grassy slope; on the other—the side facing the Stedelijk—is a grim, blank wall.

Andersson had handed Crouwel his greatest challenge. Instead of opening expansively onto the Museumplein, the new entrance would be staring into the wrong side of the donkey's ear. Worse, the people hanging out on the inclined green would have their backs to the museum. So instead of building out to the property line, Crouwel put half the new gallery space (plus an auditorium) on an upper floor and buried the other half, leaving the ground level for a plaza sheltered by an extravagantly cantilevered canopy. There is talk of keeping that outdoor area buzzing with activities. But left alone, the plaza, which is bounded by the glassed-in lobby and that unfortunate wall, seems likely to become an isolated and needless appendage to a grand civic square. Crouwel's design practically acknowledges as much: At one end of this potential dead zone, he has plunked a freestanding mechanical tower and loading dock that generally gets cropped out of photos but can't be edited out of existence.

The new Stedelijk's ungainliness is especially mystifying because few old cities have hitched their future to new architecture as decisively as Amsterdam. It's been expanding in every direction, most dramatically into the northern archipelago of artificial islands, where what were once green smudges on a gray horizon have sprouted crystalline structures. The Eye, a just-opened film museum by Delugan Meissl Associated Architects, swoops down to the water, and the future Palace of Justice by Claus en Kaan Architecten rears energetically above it. Old brick warehouses peek out from within new apartment complexes, idle piers sprout fresh neighborhoods, and 17th-century houses stand alongside modernist lofts. This is a city that takes its history and urbanism seriously.

Crouwel has tried to honor that tradition by establishing a new equilibrium at the Stedelijk Museum, balancing fussy masonry with sleek industrial surfaces, ornamental curlicues with aerodynamic curves, and earthy color with white glare. If this were a movie about shackled strangers who discover how much they have in common, it would feel unbearably contrived. The new museum is an architectural odd couple, a pair of mismatched buildings trying to ignore the fact that they are joined at the hip.

Justin Davidson, a Pulitzer Prize–winning critic, writes on both architecture and music for New York magazine.