Mr Williamson said the devices would be delivered to schools.
He also pledged to publish a remote education framework to support schools and colleges with delivering lessons during the latest national lockdown.
It comes as research says children from poorer families are likely to struggle more with remote learning.
The Department for Education said its data showed that over 700,000 devices had been delivered to schools in England so far during the pandemic - 100,000 of which were delivered last week.
The department says the additional 300,000 laptops and tablets lifts government investment by another £100m, meaning over £400m will have been invested in supporting disadvantaged children who need help with access to technology during the pandemic.
But the department has faced mounting criticism over huge percentages of pupils not having access to digital devices, nine months into the pandemic.
Mr Williamson said the DfE was "doing everything in our power to support schools with high-quality remote education".
He said: "These additional devices, on top of the 100,000 delivered last week, add to the significant support we are making available to help schools deliver high-quality online learning, as we know they have been doing."
On top of this, the remote education framework would support schools and colleges with delivering education for pupils who are learning from home, he said.
The frameworks, which are voluntary and should be adapted for schools' individual circumstances, will "help them to identify the strengths and areas for improvement in the lessons and teaching they provide remotely".
But Geoff Barton, head of the Association of School and College Leaders, said: "While we welcome the extra laptops and tablets announced, it is pretty poor that nearly a year after this crisis began we are only now inching up to the number of devices that are needed.
"The reality is that this extra provision is coming when we are already well into the new lockdown and after a heavily disrupted autumn term in which many children had to self-isolate in line with coronavirus protocols," he said.
"The government was slow off the mark to address the digital divide early in the crisis and is now trying to make up for lost time."