The ruling by U.S. District Judge Richard Kopf of Lincoln followed similar decisions earlier in two other cases.
The abortion ban was signed last year by President Bush but was not enforced because judges in Lincoln, New York and San Francisco agreed to hear evidence in three simultaneous non-jury trials on whether it violates the Constitution.
In late August, U.S. District Judge Richard C. Casey in New York declared the law unconstitutional, even though he called the procedure "gruesome, brutal, barbaric and uncivilized."
He faulted the ban for not containing an exception to protect a woman's health, something the Supreme Court has made clear is required in laws prohibiting particular types of abortion.
On June 1, U.S. District Judge Phyllis Hamilton in San Francisco also found the law unconstitutional, saying it violates a woman's right to choose an abortion.
The rulings are expected to be appealed to the Supreme Court.
"We are in the process of the appeal of these issues now, which tells you exactly what we're doing and where we're going," Attorney General John Ashcroft said after Casey's ruling.
The law, signed last November, banned a procedure known to doctors as intact dilation and extraction and called partial-birth abortion by abortion rights foes. The fetus is partially removed from the womb, and the skull is punctured or crushed.
The ban, which President Clinton twice vetoed, was seen by abortion rights activists as a fundamental departure from the Supreme Court's 1973 precedent in Roe v. Wade. But the Bush administration has argued that the procedure is cruel and unnecessary and causes pain to the fetus.
At trials earlier this year, doctors testified that of 1.3 million abortions performed annually, the law would affect about 130,000, almost all in the second trimester. Some observers suggest the number would be much lower — 2,200 to 5,000.
Doctors have construed the Supreme Court's decision in Roe. v. Wade to mean they can perform abortions usually until the 24th to 28th week after conception, or until the "point of viability," when a healthy fetus is thought to be able to survive outside the womb. Generally, abortions after the "point of viability" are performed only to preserve the mother's health.
The Supreme Court in 2000 overturned a Nebraska partial-birth abortion law because it did not allow the banned procedure even when a doctor believes the method is the best way to preserve the woman's health.
To get around that decision, Congress simply declared that the procedure is never medically necessary — and during weeks of testimony, doctors testifying for the government stressed that same point — claiming that there are better alternatives to the method, and that it may even be harmful to women.
Witnesses for the abortion providers, however, testified in all three trials that the banned method is often preferred and sometimes necessary to preserve a woman's health.
Because of the possibility that the fetus may partially exit a woman during an otherwise legal procedure, abortion rights advocates said the law could ban almost all second-trimester abortions, which account for about 10 percent of all abortions in the United States.