If convicted of the felony charge, Janklow could face up to 10 years in prison and a $10,000 fine. The House of Representatives ethics committee will also investigate.
The 63-year-old congressman, one of the state's most powerful politicians, is still recovering from injuries sustained in the crash and was not immediately available for comment. His son said the family had discussed the charges but there was no talk of him resigning.
According to the accident report, Janklow was driving more than 70 mph when his Cadillac went through a stop sign at a rural intersection Aug. 16 and the motorcycle hit the side of his car. The crash broke Janklow's right hand and he suffered a head injury. The motorcyclist, a 55-year-old farmer from Minnesota, died at the scene.
Moody County State's Attorney Bill Ellingson said Friday that the facts of the case establish probable cause for a charge of second-degree manslaughter, but not vehicular homicide, which would require the driver to be under the influence of alcohol or drugs.
In addition to the felony, the prosecutor charged Janklow with three misdemeanors: failure to stop, going 71 in a 55 mile an hour zone and reckless driving. The first two carry a maximum penalty of 30 days in jail, while the top penalty for reckless driving is one year in jail.
The congressman had been on his way home to Brandon after an event in Aberdeen and a stop in Flandreau when his car went through the intersection and was struck just behind the driver's door by a Harley-Davidson driven by Randy Scott of Hardwick, Minn.
Scott's mother, Marcella Scott, and family thanked the Highway Patrol for its investigation and Ellingson for his consideration of charges.
"Although no judge or jury can bring Randy back to us, we view the criminal charges filed today as both reasonable and appropriate," she said in a statement.
Janklow, a Republican, served 16 years as governor and four years as state attorney general before he was elected to the state's only U.S. House seat last year.
His son and his doctor have said it's unlikely that he will return to work next week when Congress reconvenes because he is still recovering.
Because he is now charged with a felony, the U.S. House of Representatives ethics committee will automatically investigate. The committee's rules say representatives who plead guilty or are convicted of a crime that carries more than two years in prison can't vote in the chamber until his or her record is cleared, or until re-elected.
If Janklow were to resign, Republican Gov. Mike Rounds would call a special election within three months to fill it.
Janklow's initial court appearance is scheduled for Tuesday in Flandreau. If he wants a preliminary hearing, one would be scheduled then and bond would also be set, Ellingson said.
A self-proclaimed speeder, Janklow got 12 speeding tickets in 11 South Dakota counties from 1990 to 1994 and paid more than $1,000 in fines. He often drove 15 mph to 20 mph faster than legal speed limits and once got caught going 90 mph in a 65-mph zone.
Janklow's son, Russell Janklow, said Friday it would be inappropriate to talk publicly about the case. He said the family had talked about the charges Friday morning and there had been no discussion of Janklow resigning.
"We believe we have a system in place that will deal with this, the judicial system, and we believe in it," he said.
Before the accident, Janklow was mentioned as a possible candidate against Sen. Tom Daschle, D-S.D., who plans to seek re-election next year.