Interstate 99 (abbreviated I-99) is a partially completed main route of the Interstate Highway System in central Pennsylvania. The current southern terminus is at exit 146 of the Pennsylvania Turnpike (Interstate 70 and Interstate 76) north of Bedford, where the road continues south as U.S. Route 220. The northern terminus is currently at U.S. Route 322 near State College, where I-99 exits onto a temporary interchange with U.S. 322. Currently, all of I-99 runs concurrent to US 220.

Unlike most Interstate numbers, which are assigned by AASHTO to fit into a grid, I-99's number was written into Section 332 of the National Highway Designation Act of 1995 by Bud Shuster, then-chair of the U.S. House Committee on Transportation and Infrastructure, the bill's sponsor, and the representative of the district through which the highway runs. The number "99" violates the AASHTO numbering convention associated with Interstate highways. Under this system, the lowest numbers start on the West Coast progressing in order to the highest numbers on the East Coast. Several north-south routes, including Interstates 81, 83, 87, 89, 91, 93, 95, and 97 all lie east of Interstate 99. This inconsistency irks many road enthusiasts, and some have expressed their disapproval of this in various forums, proposing alternative designations including I-81, I-83, I-170, I-576,I-776, I-976, or I-980. The Federal Highway Administration addresses this issue in the "Interstate FAQ" on their web site, explaining that the designation can only be changed by an act of Congress.

When construction on I-99 is completed, the southern terminus of the route will be at an interchange with Interstate 68 in Cumberland, Maryland. The northern terminus will be at an interchange with Interstate 86 in Corning, New York.

I-99 map