However, she said the disruption caused by the Covid pandemic would have an impact.
Ms Somerville was accused of scrapping the 2026 target during a meeting of Holyrood's education committee last week.
The SNP's 2016 Programme for Government said ministers intended to "substantially eliminate the gap over the course of the next decade".
First Minister Nicola Sturgeon has described improving education as her "defining mission".
However, a report last year by the Auditor General and the Accounts Commission found progress in closing the attainment gap had been "limited".
Appearing before the education committee, Ms Somerville refused to set an “arbitrary date” for closing the gap, insisting it had “always been a long-term project”.
Asked if the 2026 pledge had been abandoned, Ms Somerville told The Scotsman: "No, it hasn't.
"Obviously the closure of the attainment gap is going to be a long-term endeavour – the OECD [Organisation for Economic Co-operation and Development] has said that and so have others.
"But we do expect to see substantial progress.
"Clearly, we will have the impact of Covid to be taken account of, but no, it has not been abandoned."
She added: "We would expect to see substantial progress in tackling the attainment gap by 2026.
"Obviously, there will be the impact of the pandemic, but we would still expect to see substantial progress."
Ms Somerville said the Scottish Government was "absolutely determined to do everything we can to tackle the attainment gap – to do that as quickly as possible".
She said: "There has been a global pandemic, that will have an impact, but in essence the policy is exactly the same as the 2016 Programme for Government."
Put to her that her language appeared to be shifting away from the Programme for Government commitment, she said: "The language I'm using obviously suggests, as everyone knows, that a pandemic, a global pandemic, will have an impact on schooling.
"We know that – it's happening right across in different countries.
"But that's exactly why the Scottish Government has put in the additional investment, the additional teachers, through our local authorities into our schools to be able to assist schools with recovery.
"And that's exactly why we're still determined to see the substantial closure of the attainment gap."
Pushed on whether the target for this is 2026, she said: "Yes."
Ms Somerville made the comments while visiting Methilhill Primary School in Fife, where she met children using new micro:bit coding devices, which are being rolled out to all primary, secondary and additional support needs schools in Scotland.
The gadgets aim to boost computing science and digital literacy in schools, with all primaries receiving 20 devices over the coming weeks.
The scheme is being delivered by the Micro:bit Educational Foundation, an education non-profit organisation, following investment by the Scottish Government.
Ms Somerville said the Government was “proud to be leading the world in creating quality engagement in computing sciences among our young learners”.