Roshan, who specialised in a gas separation method used to enrich uranium, died immediately in an explosion caused by a magnetic bomb attached to his car by two men on a motorbike.
Two other people in the car were injured, according to Iranian media. One of them was his bodyguard and driver.
"The responsibility of this explosion falls on the Zionist regime," Tehran province deputy governor Safar Ali Bratloo, told Iran's Arabic-language Al-Alam broadcaster, using Iran's term for Israel.
He pointed out that three other Iranian scientists, at least two of whom worked on nuclear programmes, were killed in 2010 and 2011 by the same method.
The current head of Iran's atomic organisation, Fereydoun Abbasi, escaped another such attempt in November 2010.
Iran's parliament erupted with yells of "Death to Israel!" and "Death to America!" during a speech by one MP who said Wednesday's attack would not dissuade the Islamic republic from "achieving progress".
That message was taken up by Vice-President Mohammad Reza Rahimi on state television.
"Today those who claim to be combating terrorism have targeted Iranian scientists. They should know that Iranian scientists are more determined than ever in striding towards Iran's progress," he said.
Iran has accused Israel’s Mossad of being behind all the killings and hinted that the US was cooperating in the assassination programme.
Israel, which has its own nuclear weapons without officially recognising the fact, has threatened to bomb Iran’s nuclear facilities but at present is pushing for tougher international pressure on Tehran.
In November Iran accused the UN's nuclear watchdog, the IAEA, of putting its scientists' lives in danger by listing them in an appendix to a report.
The Danish presidency of the EU on Wednesday promised more sanctions against Iran, following a call by French Foreign Affairs Minister Alain Juppé.
The US and Europe have upped the pressure on Iran since an International Atomic Energy Authority (IAEA) report in November expressed "serious concerns regarding
possible military dimensions to Iran's nuclear programme" and relations have gone from bad to worse:
- Although Russia and China have resisted tightening UN sanctions on Tehran, the US and the European Union have taken extra measures;
- Iran captured what it said was a CIA drone in December;
- Iran this month threatened to close the Strait of Hormuz – through which 20 per cent of the world’s oil passes – if it is attacked or sanctions hit oil supplies;
- Iran has threatened use the “full force” of its navy if the US redeploys and aircraft carrier in the Persian Gulf;
- The US declared that it would continue sending warships to the area, where its Fifth Fleet is based;
- Britain has dispatched its most modern destroyer, HMS Daring, to the Gulf;
- Iran says it is about to hold naval manoeuvres, following ones that ended less than two weeks ago in which three anti-ship missiles were fired;
- Iran this month sentenced US citizen Amir Hekamti to death, saying he was a CIA spy;
- The IAEA said Monday that Iran has begun enriching uranium at a second facility.