Saudi Arabia, a key US ally and the world's biggest oil producer, has embarked on a cautious reform programme under King Abdullah who came to power last year.
Roth said: "The government seems, at least at the rhetorical level, interested in reform."
HRW made a first exploratory visit to Saudi Arabia in 2003. No other major rights groups have been able to conduct field-work in the vast desert country where there is an absolute monarchy and a religious police.
The United States and rights groups have often criticised Saudi Arabia over many issues including religious freedoms, freedom of expression and the imposition of the death penalty through public beheading.
UK-based Amnesty International is due to make its first major visit to the country in January, a spokesman in London said.
He said: "We will meet government ministers, establish contacts with civil society and visit prisons."
Saudi Arabia says its system of Islamic laws ensure full rights for Muslims and non-Muslims.
Nearly one-third of the country's 24 million population are foreigners, mainly blue-collar workers from Asian countries.