After a rare disappointment for Adam Peaty on Monday, attention switched to another of England’s swimming stars on Tuesday – and Proud delivered.
His time was slightly slower than the Games record of 21.30 he set in the heats, but was enough to take gold.
“I’m very pleased with that. Last night was about the time, but that was all about the medal,” Proud said.
“It’s been a really special night. I am very positive for the rest of the year.”
Victory brought Proud his second medal of the Games, having won 4x100m freestyle silver.
Scotland’s Duncan Scott – a surprise winner of the 100m freestyle on Sunday – won silver in the men’s 200m individual medley behind Australia’s Mitch Larkin.
Holly Hibbott won 400m freestyle silver ahead of England team-mate Eleanor Faulkner, who secured her third bronze of the Games. Neither could get near 17-year-old Australian Ariarne Titmus, who made it a hat-trick of gold medals.
Wales’ Georgia Davies took bronze in the 50m backstroke, while team-mate Dan Jervis improved his lifetime best in the 1,500m freestyle by three seconds to win silver behind Australia’s Jack McLoughlin.
In the relay events, England’s team of Peaty, Proud, James Guy and Luke Greenbank won silver in the 4x100m medley final as Australia beat them to the gold by nine one hundredths of a second with a Games record of three minutes 31.04 seconds.
Wales’s team of Chloe Tutton, Alys Thomas, Kathryn Greenslade and Davies won bronze in the women’s 4x100m medley relay.
Semenya produced a confident run as she blew away her opponents at the Carrara Stadium.
The 27-year-old, who has been subject to gender testing in the past, finished ahead of silver medallist Beatrice Chepkoech of Kenya and broke Zola Budd’s national record in the process.
Melissa Courtney won bronze for Wales in a personal best of 4:03.44.
Semenya has said she will try to break the 35-year-old 800m world record – the longest-standing in track and field – later in the week.
In the men’s 110m hurdles, Andrew Pozzi never recovered after hitting the first hurdle and finished sixth as Jamaica’s Ronald Levy won gold in 13.19 seconds
“It was an untidy race with mistakes,” Pozzi said. “I made too many and wasn’t good enough.”
There was disappointment too for England’s Sophie Hitchon as she failed to register a throw in the women’s hammer final.
Scotland’s flagbearer Eilidh Doyle progressed to the final of the women’s 400m, while British record holder Dina Asher-Smith also eased through to the next round in the 200m.
England’s Zharnel Hughes was second fastest into the semi-finals of the men’s 200m in 20.34 seconds.
England’s Kyle Langford and Scotland’s Jake Wightman progressed to the 800m final, where they will meet defending champion and London 2012 silver medallist Nijel Amos of Botswana.
More Tanfield success, tears of joy for Simmonds
Harry Tanfield joined his brother Charlie in the medals with silver in the men’s individual time trial.
Charlie, 21, had won gold in the individual pursuit and silver in the team pursuit in the velodrome last week.
Harry, 23, made it an even more memorable Games for the family as he finished 30 seconds behind winner Cameron Meyer of Australia.
Jersey’s Dan Halksworth finished 26th as he became the first athlete to compete in three different sports at three different Games.
The 32-year-old featured in swimming in 2006 and triathlon in Glasgow four years ago.
Simmonds later took England’s second time-trial medal of the day with bronze in the women’s event.
The 29-year-old, who switched from rowing to cycling in 2010 and lost 45kg in weight, was in tears as she finished.
However, there was frustration for Simmonds’ England team-mate Melissa Lowther, who was prevented from competing.
An administrative error by Team England meant she was not formally entered into the event, which left her “gutted”.