The governor signed the bill during a Capitol ceremony attended by legislators and business leaders. He received a round of applause as he completed the process.
The bill makes several changes in where lawsuits can be filed and how much can be awarded.
It was passed during a contentious special legislative session that ran from May 19 to June 8.
Trial lawyers and consumer advocates say the bill limits compensation for people who are hurt by faulty products or others' negligence.
The U.S. Chamber of Commerce last year called Mississippi a "judicial hellhole'' because of large jury awards.
Barbour was elected governor in November 2003 on promises to change Mississippi's civil justice system. He went to Washington and New York last week to do interviews with business magazines and cable and radio networks to tout the bill, which becomes law Sept. 1.
The bill caps pain-and-suffering damage awards at $1 million in most lawsuits and places a hard cap of $500,000 in medical malpractice cases.