Optimer Pharmaceuticals Inc (Nasdaq: OPTR) was a biopharmaceutical company, originally headquartered in San Diego, California, and later moved to Jersey City, New Jersey, United States.

The company focused on developing specialty drugs to treat gastrointestinal infections and related diseases.

In 2011, it received regulatory approval for the first antibacterial drug, Dificid, in 25 years approved to treat certain types of diarrhea (i.e., Clostridioides difficile induced diarrhea) in adults.

CEO Michael Nientse Chang founded, funded, and built the company with his wife, Tessie Mary Che, who was COO from 1998 to 2011.

From 2007 to 2013, Optimer Pharmaceuticals was traded on the Nasdaq Stock Exchange under OPTR.