The poll asked 237 UK tech executives whether they were positive about the UK tech sector's potential for growth over the next two years.
While 70% of UK replied "overall positive", that's still down considerably compared to 93% in March 2016, two months before the referendum.
Julian David, CEO of techUK, said in a statement:
"UK tech is resilient and innovative but this substantial drop in confidence clearly demonstrates the need to be vigilant to immediate and pressing concerns faced by tech companies. To maintain the sector's exceptional growth rate they need to be confident that they will have access to the digital single market, a talented workforce and that international data flows will be protected.
"Our members are also very clear that there is also an opportunity for the UK. Now is the time to develop a world-leading digital infrastructure, and make the UK the place to invest and grow. Government must power this digital revolution."
Some 49% of respondents said that the outcome of the EU referendum would have a detrimental impact on foreign direct investment over the next two years, while 48% said capital investment and R&D spend in the UK would also take a hit over the same time frame.
In response to the survey findings, techUK has launched a Brexit unit, headed by techUK policy director Charlotte Holloway, which will focus on mitigating the impact of Brexit on the tech industry.