The announcement was made as House of Representatives members in Washington, led by Republicans, debated whether to repeal the law. Repeal of Obama's signature domestic accomplishment would be symbolic because Democrats control the Senate and White House.
The six additional states, all with Republican attorneys general, joined Florida and 19 others in the legal action, Florida Attorney General Pam Bondi said.
"It sends a strong message that more than half of the states consider the health care law unconstitutional and are willing to fight it in court," she said in a statement.
The states claim the health care law is unconstitutional and violates Americans' rights by forcing them to buy health insurance by 2014 or face penalties.
Government attorneys have said the states do not have standing to challenge the law and want the case dismissed.
Lawsuits have been filed elsewhere. A federal judge in Virginia ruled in December that the insurance-purchase mandate was unconstitutional, though two other federal judges have upheld the requirement. It's expected the Supreme Court will ultimately have to resolve the issue.
"It is important to note that two of the three courts that have reviewed this law on the merits have found it constitutional, and those decisions - as well as two others the government prevailed on - are pending in courts of appeal. At the same time, trial courts in additional cases have dismissed numerous challenges on jurisdictional and other grounds that have not been appealed," Justice Department spokeswoman Tracy Schmaler said.
Meanwhile, the White House dismissed an expected vote on repealing the law, saying the Republicans' push was not a serious legislative effort. Democrats have said they will block repeal in the Senate.
In the Florida case, the states also argue the federal government is violating the Constitution by forcing a mandate on the states without providing money to pay for it. They say the new law gives the state's the impossible choice of accepting the new costs or, as punishment, forfeiting federal health care funding.
Florida U.S. District Judge Roger Vinson could rule later this month whether he will grant a summary judgment in favor of the states or the Obama administration without a trial.
Florida's former Republican Attorney General Bill McCollum filed the lawsuit just minutes after President Barack Obama signed the 10-year, $938 billion health care bill into law in March. He chose a court in Pensacola, one of Florida's most conservative cities. The nation's most influential small business lobby, the National Federation of Independent Business, also joined the suit.
Joining the coalition in the Florida case were: Iowa, Kansas, Maine, Ohio, Wisconsin and Wyoming.
The other states that are suing are Alabama, Alaska, Arizona, Colorado, Georgia, Indiana, Idaho, Louisiana, Michigan, Mississippi, Nebraska, Nevada, North Dakota, Pennsylvania, South Carolina, South Dakota, Texas, Utah and Washington.