Ashworth, who has recently been linked with fellow Premier League clubs Newcastle and Everton, said the new role was created due to an increase in his workload and the greater responsibility for women's football and an expansion of player welfare, plus external factors such as Brexit and the coronavirus pandemic.
"The assistant technical director will be exactly that, supporting me on a day-to-day basis," Ashworth told Brighton's website. "Since I came to the club, the brief and workload for the technical director role has expanded to include the women's and girls' football operations and analysis across both the men's and women's set up.
"We have also expanded what we are doing in terms of player welfare and psychology during the past 18 months.
"We have also seen additional workload due to various external factors such as Brexit and the new governing body endorsement procedure, and even the effect of the pandemic on the Premier League's evolving policies and procedures.
"David's football knowledge and experience is exceptional and to have his help, expertise and experience on a day-to-day basis will be a great benefit for the club."
Weir began his coaching career working with Everton's academy and reserves and had a brief spell as Sheffield United manager in 2013.
The 51-year-old then held assistant manager roles at Brentford, Rangers and Nottingham Forest before joining Albion in April 2018.