However, the former attorney-general said that when he later had to reach a final, definitive decision, he had taken "a different view".
Nadim Baba, Al Jazeera's correspondent in London, said Goldsmith's view was altered after talks with US and British officials.
"What's become clear, is that once the resolution known as 1441 was adopted ... he started to consult widely amongst people in Britain and abroad and that led him to form a more concrete view about the legality of war," Baba said.
Resolution 1441 gave Saddam Hussein a final chance to give up his weapons of mass destruction, but there has been disagreement over whether this authorised military action against him in the event he did not co-operate.
Goldsmith's evidence on Wednesday came a day after two former senior legal advisers at Britain's foreign ministry had told the inquiry that the war in Iraq was illegal.
Michael Wood and Elizabeth Wilmshurst, his deputy at the time, told the hearing on Tuesday that they believed the use of force could not be justified without a specific mandate from the UN.
Goldsmith told the inquiry that by February 2003, the month before the conflict, he had changed his position and believed using force against Iraq would be legal under resolution 1441.
"I was of the view that there was a reasonable case that a second resolution was not necessary and that was, on past precedent, sufficient to constitute a green light," he said.
Critics of the war suspect that Goldsmith was pressured by Tony Blair, the former British prime minister, to change his mind.
Goldsmith told the hearing that a letter he sent to the then prime minister in July 2002, outlining that war could not be justified without a UN resolution, was "not terribly welcome".
"I don't think it was terribly welcome. You'd have to ask Mr Blair [why], but I don't think it was welcome," he said.
Goldsmith's evidence comes two days before Blair is due to appear before the inquiry.
The issue of the legality of the conflict is likely to form a key part of his questioning.