It was far from a classic semi-final at the national stadium even though it was a wonderful strike from Maurice Edu after 20 minutes which gave the Light Blues the interval lead.
The hard-working Steelmen stormed back after the break levelling in the 66th minute through Keith Lasley but Naismith's close-range header, after good work by Nikica Jelavic, assured the cup holders the chance to defend the trophy against Celtic.
However, on this evidence, and on the evidence of the Hoops' recent form, including their 4-1 semi-final win over Aberdeen on Saturday, Rangers will have to improve if they are to retain the cup although before that, they meet Celtic at Ibrox in the fifth-round of Scottish Cup next week.
Rangers manager Walter Smith made three changes to his side with Neil Alexander taking over from number one keeper Allan McGregor, as he had done in previous rounds of the competition.
Edu and striker Kyle Lafferty replaced Jamie Ness and Vladimir Weiss respectively, while Lasley returned from injury to replace Steve Jones.
With the game televised live, the national stadium was far from full but the Motherwell fans were encouraged when Jamie Murphy had an effort on goal after a minute but Alexander saved with ease.
As the game began to slowly warm up, a searching pass from Ibrox midfielder Steven Davis sent Jelavic racing through but Well keeper Darren Randolph came racing out to clear.
However, it was a mostly uninspiring start to the game.
In the 15th minute Edu burrowed his way in to the Motherwell box before slipping the ball to Jelavic who played it back to Lee McCulloch whose drive from the edge of the box was wide of the target.
At the other end moments later, Well striker John Sutton got a close-range header in from a Chris Humphrey cross but it lacked the power to trouble Alexander.
Stuart McCall's side were working hard to contain Rangers but their task became all the more difficult when Edu gave the Light Blues the lead with a wonderful strike.
The roving Lafferty cut in the from the left-hand side and released the ball to the USA international who moved it on to his left foot before thundering it past Randolph from 25 yards.
There was hardly a rousing fight back from the Lanarkshire club, however on the half-hour mark left back Steven Hammell's corner found the unmarked Tom Hateley 25 yards from goal but the son of the former Rangers striker, Mark Hateley, blazed the ball high over the bar.
Five minutes later, after Edu had fouled Lasley 30 yards out, Hammell's free-kick got past the Rangers wall but Alexander was down to his left-hand side to smother.
Then, with Motherwell enjoying a decent spell of possession, Murphy fired a right-footed drive from distance over the bar, highlighting the lack of real threat from the Fir Park side.
All the time Rangers were struggling to get out of second gear but before the break Edu had another effort, this time from closer to goal, saved by Randolph.
The SPL champions could not get any kind of rhythm into their play which became increasingly scrappy.
John Fleck replaced Lafferty in the 64th minute as Smith attempted to get his side back in control, but two minutes later the Steelmen were level through Lasley.
As the Fir Park side swarmed around the Ibrox penalty area, Murphy played the ball in to Sutton who stretched to lay it off to the former Plymouth player who drilled it low past Alexander from 20 yards.
Rangers stepped up their efforts and after defender Madjid Bougherra won a corner following one of his trademark runs, McCulloch headed Fleck's set-piece just over the bar.
With 15 minutes remaining there was tangible relief among the nervy Rangers supporters when Naismith put the Govan club back in front with a well-worked goal.
Davis' long, diagonal pass found Jelavic wide in the left and when he moved in to the box and cut the ball back, the former Kilmarnock player had the easiest of tasks heading in from six yards.
Motherwell were clearly deflated but reasserted themselves, pinning Rangers in to their own half and with five minutes remaining Well full-back Steven Saunders came close with a header from six yards out which Alexander was grateful to save.
In the first minute of injury time Murphy failed to connect properly from 14 yards allowing the Gers keeper another easy save before Naismith missed a decent chance at the other end as Rangers ultimately saw the game out.