The Premiership club said that 31-year-old Van Rooyen will arrive at Bath ahead of the new season.
He started the Super Rugby final in Christchurch two days ago, when the Lions suffered a 37-18 defeat to the Crusaders.
Van Rooyen has made more than 60 appearances for the Johannesburg-based Lions, making his debut four years ago.
He will enhance Bath’s front-row strength and competition for the loosehead position after they lost England hopeful Beno Obano to an injury earlier this year that could see him miss the entire 2018-19 campaign.
The experienced van Rooyen said: “I’m really looking forward to testing myself in the Northern Hemisphere and playing for a Club with a great deal of ambition going into the new season.”
Bath Rugby director Todd Blackadder told the club’s official website: “We are really pleased to bring someone in of Jacques’ calibre in time for the new season.”
“He has proven himself in Super Rugby and will bring a great deal to the squad, both at the set-piece and with ball in hand.
“We have great strength in depth at loosehead with Jacques, Lucas (Noguera) and Nathan (Catt) all competing for the number one shirt after losing Beno (Obano) to injury in May.
“Jacques can also play at tighthead, which adds another option over the course of a busy season of domestic and European rugby.”
The 30-year-old back three star will move to Kingspan Stadium later this month in a similar deal to that which saw fellow Brumbies player Christian Lealiifano arrive in Belfast last season.
Speight will return to Australia on December 31, in advance of the 2019 Super Rugby season.
The Fijian-born speedster has played over 100 Super Rugby games for the Brumbies and has represented the Wallabies on 19 occasions.
He picked up an ankle injury in the last match of the 2018 season, but has been medically cleared to play before the PRO14 kicks off.
📢 BREAKING NEWS | We are delighted to confirm that Australia international Henry Speight has agreed to join us on a short-term contract until 31st December! pic.twitter.com/yG9ZlX99iA— Ulster Rugby (@UlsterRugby) August 6, 2018
“I’m excited to have the opportunity to represent a big club like Ulster,” said Speight.
“I’ve spoken to Christian (Lealiifano) a lot regarding this move and he had only great things to say about the staff, players, supporters and wider community, which welcomed him with open arms.
“This is a fresh challenge for me and I hope to embrace it by relishing every moment and by adding value to the group as best I can.
“I can’t wait to arrive in Belfast and get to work with my new teammates.”
Ulster’s Operations Director, Bryn Cunningham was pleased to be able to bolster the squad with such a quality player at this stage of the season:
“With the nature of Henry’s current contractual position, I would like to thank his local agent, the Brumbies and Rugby Australia for facilitating getting this deal across the line.”
“Henry has X factor quality and has consistently proven himself as a top performer at both Super Rugby and International level, with his most recent displays for the Brumbies being eye-catching.
“We hope he will quickly become be a real fans’ favourite at Kingspan!
“Henry will act as cover for Louis (Ludik), who sustained a hamstring injury in the final game of last season and is expected to be available for selection by November, and David Busby, who will now miss at least the first half of the season through an injury picked up in pre-season.
“His presence will also support the development of the young back three players within our senior and Academy squads.
“We saw the significant impact Christian had on Johnny McPhillips last year and we would hope that Henry will provide us with something similar.”
As a rugby lover it would be great to write a match report about a thrilling Super Rugby final, played between two evenly matched teams with sweeping backline moves, courageous defence and powerful forward play.
It would be great – but sadly that wasn’t the case.
This game resembled a great contest as much as sawdust resembles snow.
It’s difficult to remember a more one-sided final.
From the moment the Lions came up with nothing after bombarding the Crusaders line for the first eight minutes, the writing was on the wall.
And when Seta Tamanivalu crossed for the Crusaders’ opening try from their first attacking opportunity the match was as good as over.
The rest was just a dull procession, enlivened by some attacking brilliance in the second half, as we waited for Sam Whitelock to lift the Super Rugby trophy above his head – the ninth time a Crusaders captain has done that in just 23 years.
In terms of entertainment value this was like a giant python slowly strangling a small dog over an hour and a half.
You know what’s going to happen and its tedious and painful to watch.
The Lions’ attacks were about as pointless as shooting paintballs against a brick wall – and not colourful paintballs either. Grey, beige and black.
When the Lions did occasionally threaten, the Crusaders repelled their advances comfortably.
Nothing was more telling than the Lions’ attacking scrum five metres out from the Crusaders line early in the second half.
It was the visitors’ last chance to even hint at making the match competitive.
But on cue the Crusaders turned on the power and the Lions’ scrum imploded.
There was a brief flurry as Lions flanker Cyle Brink powered his way over in the 53rd minute but despite having 62 per cent of the possession and 70 per cent of the territory – the Lions were never really in it.
As sporting contests go it was one of the most one-sided finals you could witness.
A testament to the excellence of this Crusaders team, and just how high they set their standards, was that despite the match being comfortably won in the final minutes the champions defended with more venom than they had in the entire match, repelling the Lions’ vain attacking attempts with some ferocious hits.
Nothing in the end could have been more appropriate than hardworking Crusaders flanker Matt Todd turning the ball over on his own line and booting it into touch for the final act of the match.
Unfortunately the only real jubilation on the final whistle came from the players on the field.
The occasion of the final was wasted on a Christchurch crowd so weary of success that Crusaders coach Scott Robertson had to walk out onto the pitch before the match and try to rouse them to show some interest.
The AMI Stadium in Christchurch only seats 18,000 but so apathetic were fans that there were fears the venue would not be full, even for the tournament showpiece.
In the end the ground was full but so blasé were the Crusaders’ supporters about the victory that most of them had departed before victorious captain Sam Whitelock had even raised the Super Rugby trophy above his head, and well before Robertson had completed his celebratory break dance.
It is one of Super Rugby’s big problems that the nation that so dominates the tournament has fans who are so reluctant to attend a match.
There were very few sell-outs during the season with great swathes of empty seats in Auckland and Wellington, which hardly makes for good television pictures.
At Eden Park it’s understandable that crowds aren’t coming out to support the underperforming Blues but the Hurricanes in Wellington were champions as recently as 2016 and play some of the most entertaining rugby on the planet.
But for a nation spoiled by success, who have won 16 of the 23 Super Rugby titles on offer, plus the last two Rugby World Cups and 15 Bledisloe Cups in succession, even a final victory isn’t enough to raise much more than a polite nod.