Here is a list of metroplexes that overlap multiple countries:

Excluded from list is:

• City-states being part of the overlap (unless official countries are part of the overlap too). One example of that would be Rome since Vatican City is isolated within Italy with no official countries overlapping.

Between the United States and Canada

This list is from west to east:

• Vancouver, British Columbia - extends into Whatcom County, Washington
• Sault Ste. Marie, Michigan and Sault Ste. Marie, Ontario
• Port Huron, Michigan and Sarnia, Ontario areas
• Detroit, Michigan and Windsor, Ontario
• Buffalo-Niagara Falls, New York and St. Catharines-Niagara Falls, Ontario
• Ogdensburg-Massena, New York (Saint Lawrence County) and Cornwall and Prescott, Ontario (micropolitan areas)

Between the United States and Mexico

• San Diego, California and Tijuana, Baja California
• El Centro-Calexico, California and Mexicali, Baja California
• Yuma, Arizona and San Luis Rio Colorado, Sonora
• Nogales, Arizona and Nogales, Sonora
• El Paso, Texas and Juarez, Chihuahua
• Del Rio, Texas and Acuña, Coahuila
• Eagle Pass, Texas and Piedras Negras, Coahuila
• Laredo, Texas and Nuevo Laredo, Tamaulipas
• McAllen, Texas and Reynosa, Tamaulipas
• Brownsville-Harlingen, Texas and Matamoros, Tamaulipas

Elsewhere in the world

• The city of Basel, Switzerland borders both France and Germany
• Metropolitan Bratislava overlaps Slovakia, Hungary and Austria
• Aqaba, Jordan, Eilat, Israel and Taba, Egypt
• Kinshasa and Brazzaville are next to each other.
• Strasbourg, France and Kehl, Germany
• Copenhagen, Denmark and Malmö, Sweden
• Rivera, Uruguay and Santana do Livramento, Brazil
• Subotica, Serbia and Szeged, Hungary- urban sprawl between the 2 close cities
• Lille, France and Mouscron and Kortrijk, Belgium
• Metropolitan Geneva extends into France.
• Metropolitan Milan extends into Switzerland.
• Metropolitan Luxembourg City extends into Belgium, France, and Germany

Metropolitan areas